Everything. Legit. The Pokemon included, the Pokemon excluded, the new battle features, the new C-Gear, the new forms, Hidden Hollows, etc etc. just, omg it's amazing.
I think my favorite part is what Gym Leaders were included in the game.
Shizui is obviously my favorite. I don't think anyone needs to be told that. The fact that his Gym, like the other Gyms, even get their own theme music is amazing. Yes, I love me some consistency in the Gyms, but I don't mind personality. The new themes add that bit of personality. Shizui's being so calm, tropical, and definitely reminding me that he uses Water-type Pokemon is spot on. The Pokemon selection in this game for the Gym Leaders is also much better. Rather than having whatever Pokemon is closest to them, they actually have Pokemon with a little uniqueness to them.
Another favorite is the Hidden Hollow feature. It was forever and a day before I could actually get my games connected to the internet, due to my Wi-Fi not being compatible with the DS Lite programming. Even now, i am still unsure how to use it. Because of this, I never had access to the Dream World or the abilities the Pokemon got from it. Now, with the Hidden Hollow feature, I don't have to go to the Dream World. The Pokemon within are usually a random Pokemon from the route the Hollow is located in, and each Pokemon has its Dream World exclusive Ability. What's even better is that it isn't always a Pokemon, it can be an item! Not only that, but it replenishes with a Pokemon or item daily, and more than once a day, with only a few hours in between each interaction. Yay!
I am also loving the new characters. Akuroma plays a huge role in this game, and, he's the first villain in years to find out how wrong he was, and then want to make up for it. We haven't had that since Archie and Maxie! Not only that, but his team is boss. B]
Bianca returning is another plus. Her giving you your starter and the C-Gear is uber neat. Plus she looks adorable to boot!
Lastly, my other favorite thing about this game is the selection of Pokemon. As much as I hate Zorua and Zoroark, I'm glad they're included. I hate Psyduck and Golduck, but they fit nicely and actually make sense to be in the game. Even though I wanted them so badly, Magikarp and Feebas don't have much need being in this game before the National Dex. True, they're some of the most known Water-type Pokemon in the series, but that doesn't necessarily mean they have to be there. Even Dunsparce was a good choice to throw in! The increased number of Water-types was great, the addition of Eevee and Riolu is nifty, and pink, cute, lil' Flaffy being in the game makes it even more awesome.