Assassin's creed, in one word, is pretty.
The graphics are amazing and the layout's and town's are so great.
The main reason I enjoyed the game was because of all the extra detail in the background, running around the cities, climbing walls, finding flags and even escaping the guard were fun, and being able to go almost anywhere made this game really unique.
Doing a lot of the optional missions also helped with each assassination, because if you had all the optional information there were ways to create a strategy for a kill and really make things easier.
However, the missions can get repetitive, especially if you don't take the time to look around the city. Also, I also had a few problems with the frame rate, and the story isn't as indepth or as interesting as it could be. Still, there's enough so that it doesn't fall flat, or isn't confusing like some games that try and pull off a mystery.