Really, my biggest gripe with X and Y is how familiar they feel. Though neat, Mega Evolutions feel a little gimmicky, like the developer was desperate for any new ideas. It’s more like Goku going Super Saiyan (and inevitably turning back to normal) and less like, say, Luke Skywalker permanently becoming a Jedi. Mega Evolutions, however, only last for a battle (although it costs nothing to go through the length animation that activates it, so you can do it for every battle if you wanted).
Normally when a Pokémon evolves, they stay that way until they evolve again.
Other than that, X and Y added Mega Evolutions, new, powerful forms for certain Pokémon that you can activate per battle. It also introduced a new Pokémon type, Fairy, a first since the Second generation gave us Steel and Dark. The newest entries most notably added 3D graphics to a series that long depended on sprites, which helped make Pokémon feel more modern and animated.
