I've been struggling to articulate the exact problem with XY, because on the surface, even discounting the difficulty, it should be a perfectly fine game. and I guess it is. the routes and dungeons are well designed, the new mons are cool, the music is great, but what I think is missing is an integration of the new stuff into something greater than the sum of its parts. you have mega-evolution, but no kalos pokemon can mega-evolve. you have a pokemon with different hairstyles, but they don't do anything. you have hordes, but no new pack hunter to take advantage of it.
by contrast, take a look at phenomenons from generation 5. there are pokemon designed around this new mechanic. Audino can only be found in rustling grass and gives you a lot of experience, encouraging the player to check out shaking grass patches whenever they appear. but later in the game you might find Emolga instead, who is designed to make your life miserable. there's also Alomomola, who appears in rippling water and is an aquatic Audino. if that wasn't enough, Basculin has two forms: one which you'll find surfing regularly, and one that appears in rippling water.
and this applies to other mons as well. Deerling with seasons, Darmanitan with hidden abilities, and so on. this sort of design is lacking in Kalos, which makes it feel like a game with a bunch of stuff in it instead of a new world. ORAS integrates the new features far better, which is why I consider it the definitive gen 6 experience.
