I actually play it because of the story lol.
I'm only at Sequence 2 at the moment so I can't say much, but so far I think it is better than Brotherhood.
Improvements I have seen:
-You get hook-blade as primary weapon quite early in game, this is very useful, you can jump farther and climb higher, destroy scaffold to create distraction, getting pass people blocking your way by pulling them, there are many uses that I have not discovered yet.
-Ezio cannot do acrobatic moves (climb, long jump, swing, etc.) when he is low on health (1 or 2 bars left), disarming opponents will also become harder, always have a medicine ready.
-A lot of explosive variations.
-The Templars now can attack your territory, you will defend them in tower defense style mini game, you know like Plant vs. Zombies. Its easier if you see for yourself.
-Every area now has an assassin leader, they can follow you on some missions (No, not the call assassin stuff, they actually following you) they are very helpful, but beware, most of the time the game is over if they're dead.
-Blending with a crowd in Brotherhood is one of many way to keep a low profile, you better watch your back now since there will be Templar assassins disguised as civilian, they are highlighted with white outline in-game but didn't show on the map. Watch your back, they CAN insta-kill you if you're not ready.
-Ezio have beard.
Oh, I didn't see any single horse walking in the city, dunno if they are available later or not.
Graphics are pretty much the same.