Personally, I'm really happy with the amount of Pokémon they have in X and Y. It's not too little, and it's not too many, either. It's in-between, which is perfect. (: There may be a few more hidden Pokémon though, just waiting to be found out about. Three of which were already revealed by Serebii, as it's been stated in one of the posts above. Their names are Diancie, Hoopa, and Volcanion, and rock/fairy, psychic/ghost, and fire/water are their types, respectively. These Pokémon haven't actually been confirmed, but there's a few evidences in the game that help to prove their existence (such as the availability of their names on GTS). d:
One Pokémon that I really wanted to see in X and Y is an evolution for Jynx. I could understand why they decided not to do it, though. Jynx's caused a lot of controversies in the past, and if they brought any attention back to it, they might have to deal with the same issues again. I also wanted to see a Lucario evolution. Well, at least it's got a mega-evolution though, so that's good!