I'm going to sound like a broken record but...
No real postgame content. Some people say "What did you expect, another region?" but I say "No, I expected things to actually do!"
We got another Battle Station, the worst Safari Zone I've ever participated in and don't plan on participating in ever again, one town, a bunch of mega stones and some story plot about Looker.
I'm not saying the Battle Maison is bad. I'm just saying the entire post game shouldn't be based mostly around it and a few other things. It's not that interesting. The Safari Zone is a joke, plain and simple. If you know what it is, you probably know why I don't like it. Mega evolutions are not that great. I barely used my Blastionite, and when I did I only used it when I had a tough opponent like Draza's Goodra. But it didn't make much of a difference! And I don't even own half of the Pokémon that can mega evolve, so why should I go out of my way to catch, train and search for the mega stone that matches the Pokémon when it's not even necessary? Finally, they're very difficult to find. 8:00 pm to 9:00 pm for most of the stones? WHY. Just WHY. I don't see the purpose of that time frame at all. And the story plot with Looker? I'M SO BOOOORED OF LOOKER. He was in Black/White's story too, except that one actually have involvement with the original story plot: the Seven Sages. Now it's just Looker wants you to be his partner and- no. Just, no. It's boring, it's stupid, it's no.
"Oh but you can catch legendary Pokémon!" Yeah sure we can. Those Kanto birds, Mewtwo and Zygarde. We went from a thousand legends to barely five. I don't even want these legendary Pokemon. And why only Kanto? What is with this Kanto special treatment this generation? Barely any Pokemon fan likes Generation 1, other than a handful of Pokemon like Eevee and Ninetails and nostalgic reasons.
"Oh but with no post game you can restart and keep have different experiences!" If you continually restart anything it'll get boring and you'll start to dislike the game. That's not good.
The other things I was disappointed in was the big hype about mega evolutions and them not being very good, how overpowered Fairy types are, the unnecessary camera angles (especially in Lumiose City...), the amount of moves that I soon forgot about and never used or needed (Electric Terrain, Grassy Terrain, Misty Terrain, Play Nice...), the terrible pacing (it's rushed the first minute and then it takes you forever to get to the second gym the next) and a lot of the new Pokémon's designs. Gourgeist and Binacle especially.