BW is the worst series by a light year. It doesn't measure time, it measures distance! XD
1. The dub. Having duets as the opening theme was new but really didnt give the old feelings (I admit Japanese OPs weren't too good as well until Natsumeku Sakamichi) DP dub openings are way better. The episode names are just simply a reflection of the events in the episode (Rematch at the Nacrene Gym, Ash versus the Champion ==) and it makes them boring if compared to other seasons and XY. The Japanese episode titles are like that most of the time but they don't show the events so blandly like the dub did.
2. The way the show belittles Ash is just way too bad (losing 5 to 6 to Cameron, Pikachu doing nothing to a Panpour and a Snivy, going berserk against Elesa and almost losing 3 to 6 to Roxie). It pretty much made Ash a starting-out trainer like in the Original Series. Thank god it ended after the Unova League.
3. Ash's pokemon. It's good that Ash finally captures more than 6 pokemon in a region but the rotation mechanic is bad. It made some of the Pokemon completely ordinary as some of them like Palpitoad and Boldore only had their moments in ONE gym battle. They could've send some of his pokemon for training like Gliscor and Charizard or as Juniper's assistant like Bulbasaur did. We were happy when Scraggy became Ash's first dark-type pokemon but it was almost never featured in any episode at all.
4. The Unova League. Having Pikachu going 1-on-1 against Serperior was pathetic. And Pikachu OHKOing Serperior with a move combination when the former is almost down and the latter taking zero damage previously? What's that? Boldore and Oshawott were like death fodders against Cameron's Hyderigon too. and Ash used zero 'abnormal' strategies this time. Plus having a new guy with 0 personality win the league was just horrible although not as bad as the stupid uber spamming idiot in Sinnoh (Tyson at least showed some good character although Ash didn't meet him for long).
5. Iris and Cilan. We liked the sarcasm that Misty, May and Dawn gave us but apparently the writers wanted us to hate sarcasm so they increased Iris's by a mile. The idiotic kid comments. Robbing the Dragonite from Ash. Also they showed almost no soft sides of her as every anime series did to every female. The only one I can remember was her childhood in Opelucid. Cilan on the other hand didn't have what it takes as a replacement to Brock. His running gag was his connoisseur moments and was portrayed horribly.
6. Filler arcs. Was it necessary to show the bad guys rule the region after the league? I mean Kanto and Johto never showed that since Team Rocket and Team Flare are the only evil organisations standing as to date. They tried to make references to previous seasons then but failed. Ash's Charizard never gave the epicness to us that it used to have. Looker was OK, Team Rocket failed to show any character this time (they did have emotional moments of the three and I loved those episodes).
7. Dawn's return. First up I need to mention that I didn't hate her. It's just that Misty's return was epic and May's was cool but Dawn's didn't show any special moments. May vs. Dawn was awesome but Dawn vs Iris? Just that stupid Dragonite going crazy again ==
8. Every tournament that was held. They made me feel that they had nothing to do and are just wasting our time with all these tournaments. The tournaments are nothing special too. They should've shown some special ones like random tag battles Sinnoh did and Orinteering like the Battle Frontier.
9. Horrible planning. No new mechanics. No triple battles, no rotation battles (they're hard to execute but possible too). Having Ash fly from Icirrus to Virbank for a gym then flying back to Undella? BAD. They could've let Ash fight Marlon instead. Plenty of the unique locations in Unova were never featured (Cold Storage, Relic Castle, Reverse Mountain, Strange House, Marine Tube, Dragonspiral Tower, Undella Bay and many more)
10. The rushness. When the Decolore Island arc becomes my favourite part in this season you know how bad it was. The rushing factor (Ash fighting a gym every 8-10 episodes) which they revived from Kanto really didn't execute well. Based on the amount of new pokemon gen 5 brought us I originally expected plenty of filler episodes but they brought back the rushing mechanic and did it badly. The only slow part before Ash wins all 8 badges was when Ash goes from Castelia to Nimbasa.
Here you go. My record-breaking longest post to represent my everlasting hate for BW. I've only watch the first 10 episodes of XY and it's already better than it's preceedor. I mainly caught up with BW through Bulbapedia and I will probably skip this season.