Here is a big list of changes taken from Drayano's Platinum FAQ at GameFAQs.
Spoiler:
1. The Sinnoh Dex has been revamped. While previously holding only 151 Pokemon
it now goes up to 210. The new 59 Pokemon consist of all of the new Sinnoh
Pokemon that were originally only available when getting the National Dex
(such as Electivire, Gallade) etc as well as their pre-evolutions (such as
Ralts, Magmar, Yanma) and a few other welcome additions such as Houndour and
Swablu. All Sinnoh Dex Pokemon except for Manaphy, Phione, Dialga and Palkia
are available before you get the National Dex. The only new Sinnoh Pokemon that
are not in the Sinnoh Dex are the legendaries such as Heatran, Cresselia etc.
2. The ability to rebattle Gym Leaders has been restored, in a new area in the
Survival Area called the Top Trainer Cafe. The Elite Four now have a second
round ala Fire Red and Leaf Green where the Elite Four's Pokemon are several
levels higher, as well.
3. The gym order has been swapped around; though still displaying as the fifth
badge, the Hearthome Gym is now the third gym, the Veilstone fourth and the
Pastoria Gym fifth respectively.
4. Amity Square has had a revamp. Some of the gyms have also had a revamp,
including the Eterna Gym, Veilstone Gym and especially the Hearthome Gym.
5. There is some 'snow' in the area around Twinleaf Town. It has little impact
on the game, however.
6. The male and female characters (Dawn & Lucas) as well as the rival all have
new costumes, most notably being longer sleeved.
7. Nearly every Pokemon in the Sinnoh Dex has been given a new sprite, as well
as those legendaries introduced in Diamond and Pearl. This does not apply to
any others except for a few oddities such as Ninetales.
8. Some of the TMs & HMs around Sinnoh have been located; most notably, HM05
and HM04 are no longer in the Great Marsh or the Safari Zone, instead lying in
different areas. A few of the items have also appeared in new areas, most
notably the items required to evolve into the new Sinnoh Pokemon such as the
Electrizer, Magmarizer and the Dubious Disc (as well as the Up-grade).
9. Wild Pokemon on the whole have had some edits. For example, you can catch
Gligar and Nosepass on Route 206, Ralts on Route 208, Houndour on Route 214
etcetera.
10. Gym Leaders and Elite Four members have new teams as well as new levels.
On the whole, they are harder than before. New teams incorporate the newly
added Sinnoh Dex Pokemon - some of the Gym Leaders even have new 'main'
Pokemon. For example, Volkner's main Pokemon is no longer a Luxray.
11. The 'secret' part of the Wayward Cave is now accessible anytime. The rock
that originally barred entry unless you had Strength has now been removed,
meaning that cave can be explored to the end of the secret passage. This means
you can catch Gible and pick up TM26 Earthquake much earlier than before.
12. Elite Four members now have a completely unique battle background, each
one of them having a main colour theme of whatever their type is (except
Cynthia, of course, who has a purple/white mix). They look amazing.
13. Some of the Swarm Pokemon have been changed. For example, Pidgey is now
a normal Wild Pokemon on Route 229, but a swarm is needed to find Pinsir there.
14. Move tutors have been added to Sinnoh; there are three each teaching a
selection of moves infinite times. You have to pay them in shards.
15. One of the previously secret Poketch applications in DP can now be gotten
normally in Platinum (the Luvdisc one, specifically.)
16. The Veilstone Department Centre now has a basement containing a variety of
berries and Poffins, which is helpful if you want to evolve Feebas without
going through all of the trouble of making good Poffins or something. You can
also buy Lava Cookies in this basement.
17. Some new areas have been added, the most major of which are the Distortion
World which plays a large role in the storyline and the Battle Frontier, which
is similar to the frontier in Emerald.
18. A new Key Item called the Battle Recorder features. This allows you to
record either a Battle Frontier match or a Wi-Fi battle.
19. The Global Trade Station (GTS) in Jubilife has expanded to include extra
features such as allowing you to upload or download Battle Recorder videos.
There are also features allowing you to see other people's box designs or
Pokemon Super Contest decorations. There are two rooms dealing with Battle
Recorder videos - one allows you to view the most popular, while the other
lets you upload videos and find specific videos using a code.
20. The Pokemon which cannot be obtained within the game have changed. However
some Pokemon that could not be obtained in DP such as Tangela and Tropius are
now available within Platinum.
21. Platinum gives new forms, or as Nintendo spells it 'formes', to a total of
three Pokemon - Rotom (who gets five new forms), Giratina and Shaymin.
22. There are some new items such as the Gracidea Flower which allows Shaymin
to change into its Sky Forme or the Griseous Orb that lets Giratina change
into its Origin Forme when equipped.
23. The Wi-Fi room has been changed. It is now a room similar to the Union Room
in the GBA games but you can still ask to trade, battle etcetera. Entering
friend codes is also much easier. Instead of using the menu with letters and
etc to enter it with it being locked to numbers, there is a new window with
the 10 possible numbers that you can hit very easily with your stylus on the
touchscreen to enter the friend code.
24. The Old Chateau event with Rotom is now available as soon as you finish
the Galactic Hideout in Eterna City.
25. Key Items to get specific events such as the Secret Key for Rotom or the
Member's Card for Darkrai are now distributed on Wi-Fi by Nintendo, however
it is for limited times so you will not always be able to get it unfortunately.
26. You must now see all 210 Pokemon to get the National Dex, instead of 150.
Yes, this includes Manaphy. Thankfully you can see it. Phione is not in the
Sinnoh Dex so that does not need to be seen.
27. Some of the levels of event Pokemon have been changed. Drifloon is now 15,
Giratina is 47, Dialga & Palkia are 70, Regigigas is 1 (yes, level one) Heatran
is 50 and Darkrai is 50.
28. The Spear Pillar quest has changed quite a bit with the addition of the
Torn World. The Galactic Headquarters has also been revamped very slightly.
29. You are able to get your own house in the Resort Area. You can furnish it
and occasionally get visitors.
30. Partners such as Cheryl, Mira and Riley can now be battled against in the
Survival Area's Top Trainer Cafe.
31. Both Dialga and Palkia can now both be caught, like Groudon and Kyogre
could be in Emerald. It is now possible to catch Regirock, Regice and Registeel
in Platinum as well with specific conditions. Lastly, Articuno, Zapdos and
Moltres are also available, which seems random to me but it's useful.
32. The rival can be battled outside the Top Trainer Cafe every weekend. It is
possible for his level to rise and at the end his Pokemon team will actually
have higher levels than Red's team in GSC did, all of his team reaching 79+.
33. The partners you travel with also have new levels, being about five levels
higher than they were in DP.
34. Stark Mountain and Route 227 have been revamped. Route 227 now features a
few lava pools near the mountain and Stark Mountain itself has a lava-like hue
inside. The actual layout of the places remain the same. The Great Marsh has
had a complete graphical revamp also, now containing more navy blue hues than
the previous muddy brown.
35. There is now a Wi-Fi Plaza that allows you to play mini-games with your
friends. It seems quite fun.
36. Foreign Dex Entries can now be obtained for any of the 493 Pokemon, rather
than jut a select few.
37. A lot of trainers around Sinnoh have different Pokemon, many teams having
new Sinnoh Dex Pokemon to allow you to complete your PokeDex.
38. Some of the PokeRadar Pokemon have changed, for example there is no longer
any specific PokeRadar Pokemon on Route 218.
39. The mansion Pokemon have also changed slightly. Porygon is no longer there
but now Ditto is only obtainable from there, for example.
40. There are some more gift Pokemon to get other than the Riolu egg that Riley
gives you in D/P. While this egg still stays, you can now get a Togepi egg in
Eterna City, an Eevee from Bebe in Hearthome City (same place as D/P but now
you can get it anytime) and a Porygon from a house in Veilstone City.
41. Battles as a whole are speedier than D/P.
42. Some specific trainers have animations when they first enter battle -
namely, the rival, Gym Leaders, Elite Four members and partners like Cheryl.
43. HP Bars are now a sleek black instead of the usual white.
44. Surf speed is now equal to that of the ADV games, rather than D/P.
45. Special battles such as Gym Leaders and Elite Four members will have a
special VS. animation before the battle starts.
46. The Poketch has had a slight graphical remake and now the up/down buttons
actually go the proper directions rather than both going forward.
47. One or two moves such as Hidden Power and Razor Leaf have new animations.
48. Platinum possesses an entirely new game intro movie.
49. The cutscene at Spear Pillar is totally different.
50. Area signs when you enter an area are stylized according to the type of
area, ie the Lake Verity one has a lake background.
51. Moomoo Milk can now be bought in batches rather than one at a time.
52. Giratina, if missed in the Distortion World, can be caught in a totally
different area (Turnback Cave.) This is the first time you've been able to
catch the same legendary in two different places, I believe, ignoring runners.
it now goes up to 210. The new 59 Pokemon consist of all of the new Sinnoh
Pokemon that were originally only available when getting the National Dex
(such as Electivire, Gallade) etc as well as their pre-evolutions (such as
Ralts, Magmar, Yanma) and a few other welcome additions such as Houndour and
Swablu. All Sinnoh Dex Pokemon except for Manaphy, Phione, Dialga and Palkia
are available before you get the National Dex. The only new Sinnoh Pokemon that
are not in the Sinnoh Dex are the legendaries such as Heatran, Cresselia etc.
2. The ability to rebattle Gym Leaders has been restored, in a new area in the
Survival Area called the Top Trainer Cafe. The Elite Four now have a second
round ala Fire Red and Leaf Green where the Elite Four's Pokemon are several
levels higher, as well.
3. The gym order has been swapped around; though still displaying as the fifth
badge, the Hearthome Gym is now the third gym, the Veilstone fourth and the
Pastoria Gym fifth respectively.
4. Amity Square has had a revamp. Some of the gyms have also had a revamp,
including the Eterna Gym, Veilstone Gym and especially the Hearthome Gym.
5. There is some 'snow' in the area around Twinleaf Town. It has little impact
on the game, however.
6. The male and female characters (Dawn & Lucas) as well as the rival all have
new costumes, most notably being longer sleeved.
7. Nearly every Pokemon in the Sinnoh Dex has been given a new sprite, as well
as those legendaries introduced in Diamond and Pearl. This does not apply to
any others except for a few oddities such as Ninetales.
8. Some of the TMs & HMs around Sinnoh have been located; most notably, HM05
and HM04 are no longer in the Great Marsh or the Safari Zone, instead lying in
different areas. A few of the items have also appeared in new areas, most
notably the items required to evolve into the new Sinnoh Pokemon such as the
Electrizer, Magmarizer and the Dubious Disc (as well as the Up-grade).
9. Wild Pokemon on the whole have had some edits. For example, you can catch
Gligar and Nosepass on Route 206, Ralts on Route 208, Houndour on Route 214
etcetera.
10. Gym Leaders and Elite Four members have new teams as well as new levels.
On the whole, they are harder than before. New teams incorporate the newly
added Sinnoh Dex Pokemon - some of the Gym Leaders even have new 'main'
Pokemon. For example, Volkner's main Pokemon is no longer a Luxray.
11. The 'secret' part of the Wayward Cave is now accessible anytime. The rock
that originally barred entry unless you had Strength has now been removed,
meaning that cave can be explored to the end of the secret passage. This means
you can catch Gible and pick up TM26 Earthquake much earlier than before.
12. Elite Four members now have a completely unique battle background, each
one of them having a main colour theme of whatever their type is (except
Cynthia, of course, who has a purple/white mix). They look amazing.
13. Some of the Swarm Pokemon have been changed. For example, Pidgey is now
a normal Wild Pokemon on Route 229, but a swarm is needed to find Pinsir there.
14. Move tutors have been added to Sinnoh; there are three each teaching a
selection of moves infinite times. You have to pay them in shards.
15. One of the previously secret Poketch applications in DP can now be gotten
normally in Platinum (the Luvdisc one, specifically.)
16. The Veilstone Department Centre now has a basement containing a variety of
berries and Poffins, which is helpful if you want to evolve Feebas without
going through all of the trouble of making good Poffins or something. You can
also buy Lava Cookies in this basement.
17. Some new areas have been added, the most major of which are the Distortion
World which plays a large role in the storyline and the Battle Frontier, which
is similar to the frontier in Emerald.
18. A new Key Item called the Battle Recorder features. This allows you to
record either a Battle Frontier match or a Wi-Fi battle.
19. The Global Trade Station (GTS) in Jubilife has expanded to include extra
features such as allowing you to upload or download Battle Recorder videos.
There are also features allowing you to see other people's box designs or
Pokemon Super Contest decorations. There are two rooms dealing with Battle
Recorder videos - one allows you to view the most popular, while the other
lets you upload videos and find specific videos using a code.
20. The Pokemon which cannot be obtained within the game have changed. However
some Pokemon that could not be obtained in DP such as Tangela and Tropius are
now available within Platinum.
21. Platinum gives new forms, or as Nintendo spells it 'formes', to a total of
three Pokemon - Rotom (who gets five new forms), Giratina and Shaymin.
22. There are some new items such as the Gracidea Flower which allows Shaymin
to change into its Sky Forme or the Griseous Orb that lets Giratina change
into its Origin Forme when equipped.
23. The Wi-Fi room has been changed. It is now a room similar to the Union Room
in the GBA games but you can still ask to trade, battle etcetera. Entering
friend codes is also much easier. Instead of using the menu with letters and
etc to enter it with it being locked to numbers, there is a new window with
the 10 possible numbers that you can hit very easily with your stylus on the
touchscreen to enter the friend code.
24. The Old Chateau event with Rotom is now available as soon as you finish
the Galactic Hideout in Eterna City.
25. Key Items to get specific events such as the Secret Key for Rotom or the
Member's Card for Darkrai are now distributed on Wi-Fi by Nintendo, however
it is for limited times so you will not always be able to get it unfortunately.
26. You must now see all 210 Pokemon to get the National Dex, instead of 150.
Yes, this includes Manaphy. Thankfully you can see it. Phione is not in the
Sinnoh Dex so that does not need to be seen.
27. Some of the levels of event Pokemon have been changed. Drifloon is now 15,
Giratina is 47, Dialga & Palkia are 70, Regigigas is 1 (yes, level one) Heatran
is 50 and Darkrai is 50.
28. The Spear Pillar quest has changed quite a bit with the addition of the
Torn World. The Galactic Headquarters has also been revamped very slightly.
29. You are able to get your own house in the Resort Area. You can furnish it
and occasionally get visitors.
30. Partners such as Cheryl, Mira and Riley can now be battled against in the
Survival Area's Top Trainer Cafe.
31. Both Dialga and Palkia can now both be caught, like Groudon and Kyogre
could be in Emerald. It is now possible to catch Regirock, Regice and Registeel
in Platinum as well with specific conditions. Lastly, Articuno, Zapdos and
Moltres are also available, which seems random to me but it's useful.
32. The rival can be battled outside the Top Trainer Cafe every weekend. It is
possible for his level to rise and at the end his Pokemon team will actually
have higher levels than Red's team in GSC did, all of his team reaching 79+.
33. The partners you travel with also have new levels, being about five levels
higher than they were in DP.
34. Stark Mountain and Route 227 have been revamped. Route 227 now features a
few lava pools near the mountain and Stark Mountain itself has a lava-like hue
inside. The actual layout of the places remain the same. The Great Marsh has
had a complete graphical revamp also, now containing more navy blue hues than
the previous muddy brown.
35. There is now a Wi-Fi Plaza that allows you to play mini-games with your
friends. It seems quite fun.
36. Foreign Dex Entries can now be obtained for any of the 493 Pokemon, rather
than jut a select few.
37. A lot of trainers around Sinnoh have different Pokemon, many teams having
new Sinnoh Dex Pokemon to allow you to complete your PokeDex.
38. Some of the PokeRadar Pokemon have changed, for example there is no longer
any specific PokeRadar Pokemon on Route 218.
39. The mansion Pokemon have also changed slightly. Porygon is no longer there
but now Ditto is only obtainable from there, for example.
40. There are some more gift Pokemon to get other than the Riolu egg that Riley
gives you in D/P. While this egg still stays, you can now get a Togepi egg in
Eterna City, an Eevee from Bebe in Hearthome City (same place as D/P but now
you can get it anytime) and a Porygon from a house in Veilstone City.
41. Battles as a whole are speedier than D/P.
42. Some specific trainers have animations when they first enter battle -
namely, the rival, Gym Leaders, Elite Four members and partners like Cheryl.
43. HP Bars are now a sleek black instead of the usual white.
44. Surf speed is now equal to that of the ADV games, rather than D/P.
45. Special battles such as Gym Leaders and Elite Four members will have a
special VS. animation before the battle starts.
46. The Poketch has had a slight graphical remake and now the up/down buttons
actually go the proper directions rather than both going forward.
47. One or two moves such as Hidden Power and Razor Leaf have new animations.
48. Platinum possesses an entirely new game intro movie.
49. The cutscene at Spear Pillar is totally different.
50. Area signs when you enter an area are stylized according to the type of
area, ie the Lake Verity one has a lake background.
51. Moomoo Milk can now be bought in batches rather than one at a time.
52. Giratina, if missed in the Distortion World, can be caught in a totally
different area (Turnback Cave.) This is the first time you've been able to
catch the same legendary in two different places, I believe, ignoring runners.