The overall combat for ac3 gas been greatly improved, focusing on the ability to keep moving while killing and the new addition of the bow. The new engine make the game look top notch as well.
My biggest peeve with the game would be the boring story and how bland the new protagonist is. They have also removed the bombs and base defense system from revelations so yeah, if you liked them then oh well but if not then cool.
Overall I really liked the game but it probably wasn't the best assassin's creed in my opinion. If you liked revelations and brotherhood for the combat you will probably like this one but if you're in for the story telling then maybe not so much.