I don't like how Mega Mawile isn't available until postgame and the overworld lacks the 3D effect (not that I'd use it, but it's a very glaring oversight)
Also I noticed some slowdown in some pokémon battles, especially Double and Triple battles.
I actually liked how there wasn't much of a focus on the legendaries considering there have been far too many of them in gen 4 and 5. It was something I felt was quite refreshing.
Honestly the lack of legendary pokemon really is a let down, it actually makes me angry that they make the pokedex so big but you cannot obtain more then like 6 of the legendaries seriously? the only legendares that i can find record of anywhere on x and y and your alecated ones for your version, zygarde, mewtwo, and 1 of the birds depending on your starter, but the fact there isnt any of the like 30 others in the game.. would of made the game immensely better..
Biggest disappointment? Probably the fact I can't even play it. I mean come on! I can't go out and buy the newest Nintendo model every time pokemon gets a new game! I sincerely wish they would make a model for the DSi. I wanna get this new game, but considering it wouldn't even work on my model, I don't see any point (nor should I! :P). I do believe however it would bring in a lot more money if they made a version available for the older models like mine. It would be a win win. Anyone agree?
My biggest disappointment is that there is no MegaFeraligtr, other than that the game has been way too enjoyable for me to notice the negatives.
Well there is one negative I didn't mention, when I battled my cousin today I didn't really like the battle stage, it was too dark. I would have preferred if it was like the ones we saw in the Pokemon Smash double battles.
It will probably be an event mega. I think there was something said about most events dealing with megas or something like that.
I remembers my other disappointment I forgot earlier.
Repels no longer keep pokemon away 100%. They only lower the chance.