Positives:
I do have to say the 4th Gen Pokemon were really fun and there was not only strong ones and the cutesy ones but also the number of evolutionary relatives to past Pokemon that were introduced was what made Sinnoh stand out. The starters never disappointed me and it was our first group of starters that all got a second type by at least their final stages and also Team Galatic's plot was definitely something different than most previous villains, and the Gym puzzles were kinda fun too. And not to forget it had the female player character who inspired my own PC username.
Negatives:
The main problems, specifically with Diamond and Pearl was the sluggish speed, a.k.a. "snail's pace" especially when surfing. God it annoyed me, and also the terrible Pokedex distribution, in D/P there was a severe lacking in certain types, with Fire being the most egregious one. Because if Chimchar wasn't your starter, you'd have to settle with an underwhelming Ponyta as your Fire-type. Even the Gym Leaders and Elite Four's teams did not all contain their specialty type, like Flint and Volkner. At least Game Freak learned their lesson and fixed all these problems with Platinum.