Saying Assassin's Creed is a game about assassination is like saying Half-Life is a game about the G-Man. Oh sure, that sort of thing shows up from time to time, but it's not really a central part of the experience in terms of total gameplay hours.
I always have trouble mapping my own gameplay to the story. The main character is supposedly this angry, brooding killer, yet my version of Altair seems to be a man with many whimsical hobbies and an out-of-control case of ADD. How else can you explain an assassin who climbs the same tower a half dozen times because he just loves diving off of it? Between playing hide-and-seek with the guards, riding horses, finding flags, and base jumping, it's really hard to find the time to shiv the odd corrupt official. I actually find myself procrastinating at the actual assassination part of the game. Altair is enthusiastically energetic, yet meandering and distracted. He's like the Calvin & Hobbes of killers.
I also like how Altair is supposed to be blending in with the crowd, yet the guy has more cutting tools than the kitchen aisle at Wal-Mart. He dresses in bright white, leaps around on rooftops, and dives from the most visible points in the city. It seems like that sort of stunt would raise him to the status of local celebrity fairly quickly.
When you actually do get around to offing one of your targets, they usually give this long speech / history lesson while you wiggle your knife between their ribs. Just once I want to have one of these guys look up at me and say, "Hey! The roof diver! Man, I love your work!"
But these things are what makes the game fun. You frolic around a city, amusing yourself with bits of jumping and random violence, and when you're sick of the place you murder the guy in charge and head for the next town.
Shamus Young is a programmer and writer by trade, videogame nitpicker by inclination. If you have the patience for more of his ramblings, they can be found at ShamusYoung.com.