Basic Plot and Gameplay
I suppose I should preface this section with a disclaimer that there will be some minor plot spoilers on this page. However, I emphasize the fact that these are minor spoilers -- most of this information is readily visible if you visit the Assassin's Creed website, and you will certainly discover within the first 10 minutes of playing the game the major "spoiler" I'm about to discuss. If you want to be completely surprised, however, feel free to skip the next several paragraphs.
The majority of Assassin's Creed takes place around 1200 A.D. -- 1191 A.D. to be precise -- but your actual character is merely reliving memories of an ancestor using future technology, a device called the Animus. So there are actually two characters, Desmond Miles from 2012 A.D. and Altaïr from the time of the Crusades. Both, incidentally, are assassins, though you will spend the vast majority of the game in Altaïr's shoes. The short summary is that some people are looking for information from Altaïr, information that is part of Desmond's "genetic memory".
There's something of a Matrix vibe to the plot, but without all the machines taking over the world business. The futuristic pseudoscience seems to be at least partly a gameplay mechanic as it keeps you from dying and reloading; instead, you "lose sync" with your ancestor's memory and return to the last "sync point". "A checkpoint save system by any other name would smell as sweet…." Over time, as Desmond and Altaïr become more in sync with each other, Desmond recovers additional memories, improves "health", and improves in his abilities.
This is a gross oversimplification of the plot, so if the above summary makes you think that the game isn't something worth trying, let us state that we did find the game quite enjoyable and the storyline was compelling. It's not something likely to win any Academy Awards -- although a movie based off the story is apparently in the works -- but it does keep you playing to find out what happens next. By the end of the game we were a little disappointed that there wasn't more of a conclusion; Assassin's Creed 2 looks to be inevitable. In fact, this is supposed to be the first entry in a trilogy of games.
Gameplay (no more spoilers)
Given the title and setting, it should come as no surprise that the game involves killing... lots of killing. Throughout the game, you are given assassination targets -- men who through their actions are causing problems and need to be disposed of. However, you can't just jump straight to the major assassination target; first you have to scout out the area, gain information about the target's movements and whereabouts, and "plan" when and where to perform the assassination.
The first assassination mission is likely to impress you with the environment and gameplay. Crowds of people move throughout the city, and you can use them to blend in and escape notice. You can run across the rooftops, leaping from building to building and grabbing onto ledges, and you can climb up to higher viewpoints to get a good look at your surroundings (and reveal nearby missions). Missions consist of information gathering as well as helping out citizens who are being harassed by corrupt guards. The first few hours of the game were extremely entertaining, and you can really get into playing the role of a stealthy assassin.
After a while, flaws in the gameplay start to become more apparent. The basic pattern listed above repeats itself throughout the game: show up in a new area, do a bit of scouting, perform a few side quests, and then take out your target. Afterwards, you need to escape from the city guards and make your way back to the Assassins' Bureau. Your sword fighting skills and other abilities increase over time, and you gain access to a few additional weapons. To counteract that, guards also become more adept. However, there's no getting around the fact that the gameplay eventually becomes extremely repetitive. This is probably the biggest complaint about the game, and it's a valid point, but it's compounded by other flaws.
First impressions are that you are part of a living, breathing, teeming world -- not unlike the world found in Elder Scrolls: Oblivion. Citizens wander the streets, you can overhear vendors hawking their wares and other townsfolk engaging in gossip, and soldiers patrol the streets keeping order. Unlike Oblivion, however, all of the activity you see is merely a façade. The reality is that all the people are in scripted loops, endlessly repeating their activities. Stand in one place long enough, and you'll watch the same group of five women carrying jars on their heads walk by again and again, the same guards on patrol, etc. There are no day and night cycles -- everything takes place during the day -- which seems a little odd for an assassin game.
Furthermore, when it comes right down to it, Altaïr is virtually invincible by any competent player. Guards attack one at a time, politely taking turns as you dispatch them, so combat quickly becomes a routine affair of blocking and counterattacking. In that sense, AC is nothing like the Thief games where Garrett was highly vulnerable and dependent largely on stealth in order to survive. You can play AC in a stealthy fashion if you want to, but there's little to keep you from sprinting through town to get from place to place as quickly as possible. The only requirement is that you have to rid yourself of any pursuit before you can start/complete any of the missions (and certain missions do require you to maintain anonymity).