-Fight revamped, multiple types of ennemis, lot more weapon
-Grab system, disarm and counter isn't always easy because each type of ennemi must be beaten with different strategies
-Economic system - upgrade Ezio with armor, weapons, items, dye your clothes
-Character progression is much more present and evident
-No more investigation, it's entirely story-based with "mission givers" character just like GTA4
-Full of extra stuff do to that actually matter for something. (gathering money, upgrading your villa, collecting lore, etc)
-Villa upgrade (kinda a fable gameplay here)
-Catacomb levels (optional areas where it's more Prince of Persia style platforming). They give you a very nice bonus too
-Better faster climb
-More crowd reactions/behaviors
-Smoke bombs
-All the types of missions you see in GTA type games (chase, follow, fetch, assassinate, collect, race, etc)
-Blend is completely revamped
-You can hire different kind of friendly NPC to help you like Mercenaries, Courtesans and Thieves and you command them stuff to do.
-You can swim
-Cities are not similar like the first game
-Country side IS useful and there's even missions in it.
Seriously, there's alot of new stuff. It really is what the game ever should have been.