Conspiracy + Free-Running + Stealth = WIN!
I will say that the game has great graphics, and the camera does a great job of keeping up with the action... which is important, because you'll be turning corners of buildings,and jumping from building to building--at full speed if possible.
The combat is great--it can be complex, but it's pretty simplistic. In most cases, there is more than one way to kill an enemy, so similarly to MGS4, the entire game can cater to your play style.
There are phenomenal puzzles scattered throughout the game, most detailing real events history and somehow trying to interweave it into the Assassin's Creed world. Although very conspiracy-like, the theories that the game presents are not completely far-fetched and can leave anybody wondering.
The soundtrack is just right, it captures the ambiance of the Renaissance Era very well. The music coupled with the background of the city can very well make you forget you're not actually the player that is being controlled, but rather in a simulation.
All in all, a great title, even a must have. I could write more, but these are the most important points.