I really enjoyed the Naval Battles and was impressed with the handling and the satisfactory cheer from the crew at the end of a battle.
Some of the 100% sync requirements were a bit challenging, not to mention "The Giant and the Storm" really frustrating.
Let me give an Honorable Mention to the hunting in the game as well. Cabela's hunting could take a few notes on this one aspect of AC3.
As I've now played the majority of the game, I can see why this game is so great. Very good game.