Like a lot of people have said, the lack of post-game activity was quite disappointing.
For example, we had two random legendaries to catch and all of a sudden the three kanto birds are running around and woop woop Kiloude Town.
To me, it didn't really make much sense.
What the hell is Zygarde doing? I guess, sure, it'll get explained in Pokémon Z, but it has no explanation for why it's there. None at all. It's just there.
Same, actually, goes for Mewtwo. Mewtwo makes even less sense than Zygarde.
Why the hell is a Legendary Kanto Pokémon just hanging out in a cave in Kalos? Isn't Kalos meant to be ages away from Kanto? It honestly feels like it was just thrown there for another Legendary to catch, and they gave it two Megas to make it seem relevant.
Same goes for the Legendary Bird trio. I have no idea why they're running around Kalos. Nobody even tells you about it, either.
At least in Platinum Oak showed up and was like 'oh how strange articuno zapdos and moltres are running around go have fun catching them'.
The other thing I wasn't so keen on?
The Rollerskates. They were cute as a novelty, but I kind of expected them to be used more. I mean, I used them like once to get an item that I sold seconds later. It's a nice gimmick, but more often than not they ended up annoying me because I couldn't get into a doorway, and it was slightly uncomfortable using the D-Pad.
More Kalos Pokémon.
I was actually really disappointed with this. I went to Nuzlocke this, and I found that I had precisely one Kalos Pokémon half-way through. My starter. I don't know if this was just bad luck or whatever, but I bought X&Y to train Kalos Pokémon. Not the Kanto, Johto, Hoenn, Sinnoh and Unova Pokémon (that I do love, don't get me wrong). Plus, the game was...easy. I don't know if I'm getting older, smarter and better, but this game was just...easy. I remember in other games having to buy 20 revives and 20 hyper potions for the E4. I did the same here. I used about 5 of each. Throughout the whole thing.
Also, I felt like the whole Mega-Evo thing was a bit passive. I didn't feel any need at all to Mega-Evolve any of my Pokémon at any point. They were just strong enough without it. In all honesty, I would have preferred if we weren't able to get any Mega-Stones until post-game, and there would be some high-level Battling place that would have Trainers who used Mega-Evos. Also, the whole 'research' thing felt a bit redundant. I don't think we ever actually did any research.