Well, first off: they ruined nearly all of the old (first 251) pokemon with the new sprites. I mean, all of them have ugly colours that don't fit, (politoed having red cheeks instead of his original yellow?) or just look butt ugly and not very interesting compared to earlier sprites... (Snorlax, Aerodactyl, Espeon, Typhlosion and almost everyone else)
Everything is too slow, battling, surfing, trading etc.
The fact that you can't do random battles online is so annoying, friendcodes totally destroy it. I mean, in Japan, Pokémon Crystal had random online battles through the mobile adapter, and the current generation can't? BS
The move animations are so lackluster, even compared to GSC animations! GSC aerobeam/sacred fire for example looked KILLER, now they just look crappy, hyperbeam aswell. M&M beam FTL.
The new legendaries, I don't like any single one of them. What happened to the awesome legendaries like Raikou and Mewtwo? Arceus is just pure fail, all the other legendaries aswell.
Well, there's alot more, but the thing that bothers me the most of all is the sprites for the old pokémon being lousy and ugly.