The story in Assassin's Creed is quite complex and twists a lot, so spoilers will be avoided as much as possible. You'll play the part of Altair, a member of a creed of assassins that lived during the Crusades and is active at the time of the game (1191 AD). Altair is a high-ranking member of the creed -- for the first couple minutes. As luck would have it, the game begins just as Altair is being a complete ass to everybody and is quickly demoted, which result sin him being stripped of all his cool weapons. You'll spend the rest of the game working to regain these weapons, and along the way will be tasked with performing assassinations in order to regain Altair's rank as well. It's impossible to avoid spoilers beyond this point, so that's enough story discussion.
It's also impossible not to notice the graphics, because they are simply stunning. The cities are incredibly well designed, and it's never too obvious that a structure is climbable. Speaking of which, Altair is a pretty acrobatic guy, with the ability to climb almost everything. Oftentimes, climbing and jumping are the fastest ways to get around, and in fact, you'll be required to climb to some of the highest points in the cities in order to discover parts of the map. This is a sensible idea at first, but it gets old and tedious very quickly.
As an assassin, you'll obviously have to make sure your targets sleep with the fishes. You'll have a few different methods in which to effectively do this. Altair has access to four different kinds of weapons, namely throwing knives, a short blade, a classic sword and a hidden blade for that murderer on the go. Throwing knives are one-hit kills if you have steady aim, but assembly-line henchmen are the only ones who are dumb enough to be caught unaware by them. The short blade and sword are used when you are outnumbered, which happens a lot more often then is really necessary. Both weapons naturally have different gameplay traits, as the short blade is quick but has weak guard whereas the sword is slower but more powerful.
Altair's favorite toy, though, is the hidden blade that he keeps secured in the secret compartment in his hand. Yes, a secret compartment in his hand; Altair's ring finger was cut off as a ritual to join the Creed. With the loss of a finger, he is like Mickey Mouse, but unlike Mickey, he frequently uses his hidden blade to assassinate targets, as it is the easiest way of doing so. Switching weapons is also easy, as you'll simply use the D-pad to do so on the fly.
Aside from the D-pad, the controls are numerous but the learning curve is surprisingly quick, as the buttons are conveniently placed in the same positions as the part of Altair's body they control. For example, the bottommost A button (Altair's legs) controls whether your running or walking, while the X and B buttons (the arms) control shoving, punching, assassinating, high-fiving, etc. Finally, the Y button (the head) controls first-person view AKA "Eagle Vision." In addition to all this, holding the right trigger engages high-profile versions of the various actions. This means you'll be less discreet, such as sprinting across town or shoving people out of your way. And while the tough stuff may be fun, the guards aren't fond of rabble rousers.
The guards you'll encounter most often aren't all that bright. If your cover gets blown, you'll have to break the line of sight with the guard, usually with some acrobatics, and hide until the guards quickly give up and hope the problem resolves itself. However, although the guards may not be Einsteins, they will remember you, so leaving the area after murder is often a good idea.
If you're hungry for Xbox Live Achievements, I've got both good and bad news. The upside is that you'll get a good chunk of them by just playing through the game and killing lots of guards. The (very) bad news is that this is a sandbox game, which means (ugh) collectibles. Lots of them. You'll have to scour the cities for hundreds of flags and Altair's rivals, the Templars, in order to get all 1000 points.
Since Assassin's Creed is an open-world game, this sort of scouring and battling might not seem like a bad thing, because the genre lets gamers do whatever they feel like. Want to push people around and start fist fights? Go right ahead. Feel like starting a panic by throwing a dead body into a group of people? Go crazy. Curious to see exactly how many guards are in the square, and all you have is your sword to do the counting? You can do that, too. People will realistically react to your actions, such as calling you crazy when you push poor people around, or even helping the guards by throwing rocks at you when you're trying to climb.
But with a fast-paced combat system that makes you duck and counter-move frequently against hordes of enemies, you'll grow tired of the combat very early on. Which brings me to my biggest complaint: the game doesn't punish you for not being stealthy. I know the idea of actually wanting punishment sounds odd, but bear with me. The game is called "Assassin's Creed," isn't it? I was led to believe that meant I was supposed to plan my assassinations and be the silent killer. However, every target can be killed by waving your sword above your head, running up to them and hacking them to pieces. You can then choose to either fight the horde of guards hanging around or run away and hide. Some targets even require you to hack and slash your way through waves of hapless cannon fodder to get the kill. Where's the tact in that?
Assassin's Creed is innovative, pretty and provides plenty of fun during the Crusades. In that nuts-and-bolts sense, it lives up to expectations. But as a finished and complete product, the actual combat and confusing story keep this game from assassinating the competition.