Quiet a lot of new features.
Two on Two battles for one. Monsters can equip little power-up items and berries to use when they get into certain situations. Lots and lots of new abilities (I didn't recognize most of them when I started) and an almost completely new set of monsters to recruit. A lot of new outside of battle skills. There is a lot of stuff to do besides the obvious battling and catching... You can plent berries, build little houses, Contest (weird mini-game), making Poke'block (another weird mini-game), breeding...
You don't have to like or know anything about it.
The game is just fun.