Personally, I'm most excited about the new Pokemon that feature different looks based on their sex. With the exception of first generation's Nidoran, the differences between male and female Pokemon have generally been light cosmetic touches — longer whiskers, different colored spots, that sort of thing. Bulbapedia has compiled an awesome list of those differences, none of which are as drastic as the ones coming in Pokemon X & Y.
Just look at these Meowstics. The one with the dumb look on its face is definitely the male, just as it is with cats.
Female Pyroars have magnificent hair, while males sport gorgeous neckbeards.
Not only are the genders physically different, they learn different powers based on their gender as well. Male Meowstic learns the psychic-type move Miracle Eye, while the female version learns Extrasensory.
Then there's Furfrou, a Pokemon players can groom to their liking, transforming it from a majestic beast into a poodle with hot pants or legwarmers.
Sadly, the game's trainer customization options are sorely lacking hot pants.
I will spend hours dressing myself in this game. I am not even kidding.
Meanwhile, starters Chespin, Froakie and Fennekin are evolving into Quilladin, Frogadier and Braixen...
...changing my mind about which starter I plan to play. I spent weeks making that decision, and now I'm leaning towards the frog.
Here's a glamour shot of Mega Mewtwo X:
Nice shoulder pads.
Mega Garchomp makes his transformation much more gracefully.
There's more, like the new fossil-born Pokemon seen in the video, but I'm sure if you hang around the Pokemon X and Y website long enough you'll see more of those.
For now, I leave you with a lovely parting gift — purple-haired Celosia, orange-haired Aliana, tgreen-haired Bryony, and the blue-haired Mable, four of Team Flare's scientist leaders, looking as if they stepped right out of an 80s sci-fi music video dream. I know what my next cosplay is going to be.