The best aspects of your favorite games...combined into something unique.
Let's look at the negatives first: This game is ridiculously easy. I am not a skilled gamer, but especially after you learned the "Block" move you will hardly lose any fight - and when your health bar is low you can just run away and either wait for it to automatically go back up quickly or hide somewhere until there is no danger in walking around anymore. I had no problems whatsoever beating the final boss fights. Actually, the very first boss fight was the hardest for me.
The camera can quite literally be a pain in the ass sometimes. In the middle of fights you will see something blocking your view, after reaching an achievement of some sorts(for example saving a citizen from guards) the camera is first locked on the person you saved for far too long and then does funny things for a few seconds - all while you have to quickly get away so no guards will spot you at the crime scene.
The game is repetitive(you have to visit the same 3 cities 3 times each, every time you gain access to a new area but still have to visit the old ones at least twice, every time you fulfill similar tasks before assassinating your target, which, while the setting is very different every time, works the same way, by you first sneaking in, then watching a short scene, then assassinating your target any way you like before you have to run away from a bunch of guards(or kill them all)) and the controls, while smooth most of the time, sometimes make you jump into your own death(why do Assassin's die when touching water?) or don't interpret your directions correctly in the middle of a fight. But the latter is usually forgiven and the former just will see you set back to a point you were at a few seconds earlier.
Loading sometimes takes a bit long and the game's ending disappointing(story-wise).
Those sound like a lot, but are all really only minor things I noticed once or twice. The pros completely overshadow them.
Assassin's Creed puts you right into a beautiful, vivid world, that never feels like a game. The cities were crafted for climbing and running, but never feel or look like that. They look authentic. And jumping through them is fun.
You are in the middle of not one, but two intertwined stories that will both make you feel and doubt with the character as you progress more and more. In both stories you are a loner in an unfortunate situation, a prisoner and someone questioning their ideals while having to assassinate opponents of not him personally, but his master's.
Combat is a lot of fun and never feels too much - and you always have different ways of approaching situations. Would you prefer to sneak up and stealthily kill your target only to vanish between the masses or go for a flashy kill that will alert 20 guards around you and launch you into a big fight or a chase through an ancient city? Both equally possible, both equally valid.
Every single target - of which you have to kill 9 in the main story excluding the final boss fights - does not just have a personality, but also does something to make both your character and you as player wonder if your actions are just - and what is just in general. As the story progresses you will doubt your own actions, your order's actions and your master's goals more and more. And it will feel authentic.
Overall Assassin's Creed gives the player a lot. But there is much, much more potential in the series than shown in this game. I am looking forward to being sucked into the worlds of the later games of the series - and regret not having bought this game much, much earlier.