Well, I've played about fourteen hours so far -
The pacing for leveling is much faster, but the pacing for progression appears to be a bit slower. As stated, I'm fourteen hours in and I've just obtained the third of eight Gym badges, with most of my party being in the low-to-mid 30's. This is because you get the EXP. Share pretty early on and it acts similarly to how it did in Gen. I by giving the entire party experience instead of being equipped to a single Pokemon, though you can turn it on and off at will. In addition, you gain experience even after capturing Pokemon. Also, I might just be slow, because I've spent a fair amount of time exploring and just running through the grass training a bit and looking for Pokemon to capture.
There's a massive variety of Pokemon to capture. There's been Pokemon spilling out my ears here, I even caught a Bagon pretty early on, the earliest I've ever seen a pseudo-legendary in the wild. Additionally, while it's certainly mostly down to personal preference, the Gen VI designs I've seen so far have been vastly better than anything out of Gen 'Gurdurr' V. Litleo is one of the most adorable things I've seen in a long time, and while the starter evolutions are a bit lackluster, things like Talonflame, Meowstic, Tyrantrum, and Aurorus look pretty damn awesome.
It adds (I believe, at least) hordes of wild Pokemon, in which you're going five-on-one with lower leveled Pokeymanz and can't capture until there's only one left. Also Fairy types, roller skates for fully analog movement with the 3DS control stick, four items bindable to the usual "Register a Key Item for quick use!" button instead of the normal one item, and little things like Pokemon-Amie for playing with your 'mons or "Super Training" which I believe lets you boost their EVs or... something. I'll have to look that up. But they're little minigames almost reminiscent of something that might have been in Pokemon Stadium or something, and a cute little distraction for those of us who aren't competitively-minded.
Far as I can tell there really isn't anything drastically different from a gameplay perspective, but this is Pokemon. For what it's worth, I loathed Black with all of my being and am heartily enjoying Y, just because the move to 3D from sprites and the massive roster of old and new Pokemon alike gives it that new shine I felt the franchise kinda needed.