This game is pure genius. There is not a single aspect of it that lets it down (not even the system specifications?)
Anyway....
When I first installed and began playing the game I was just amazed by everything in it. It uses hardware to it's fullest potential and therefore still maintains a huge level of graphical detail even on slow computers. Once I was finished ogling the visuals I was drawn in by the level of interactivity you seem to have with your surroundings (not the people however). The way that you can simply run/climb/jump over anything in the game (with only a few times in the entire game when you could obviously see the developers hand in making something traversable) just draws you in; making you want to just run around just to see if the level design is really as good as it appears (it is). From then the next thing you notice is the incredible attention to detail that the developers used when making the game realistic. Gone are the days of clunky transition animations while you watch your character get off a ladder. No matter what bizarre route you decide to take when climbing or running the game controls each limb onto a realistic looking surface.
Once your vision has come to terms with the visuals then you begin to notice the intricacy of the sounds and the subtlety with witch they are employed. I can still remember the days when there would be an MP3 or similar file in the install somewhere that played on a loop when your outside to simulate voices however in Assassins Creed it seems as though each sound is sourced from a particular person, from guards (with whom you will 'interact' with many times) to mad men and drunks raving madly at you. For these reasons I think that Assassins Creed has to be one of the best games of the year if not the decade.
The only thing I disliked about Assassins Creed was it's unfilled potential. It had such a great engine that seemed to be so flexible at times it is a wonder to me why they did not incorporate more GTA style game play. By this I do not mean random pointless violence by all the extra features that it comes with e.g. greater range of weapons (like maces or bows). Once you have finished the game there is very little extra you can do other than trivial tasks like finding flags or redoing levels. Some form of currency also would have been a great bonus to the game and could have added a huge depth of interaction with the people of Assassins Creed.
That said, Assassins Creed is already set to be a legendary game in the same league as GTA or Half Life I just think that, without much extra work the game could have become a much more immersive and fun to play (if it is actually possible) than it is already