The bad news: we only got to play one match, and it's not clear just how durable the gameplay is. But as a first time experience, it was quite impressive as a feat of multiplayer game design.
Your job is to find and kill them, carefully following your detector while you move closer and closer to your target. Once you're on top of them, it's up to you to search the crowd -- not only do you have to find someone who matches their model, but you have to distinguish them from the AI characters walking around. If the person you're looking for bolts, you know that's your human target and the chase begins.
Reach them (or simply sneak up on them), and you can hit X to assassinate them. There's no fighting, no combat, and no shooting -- it's simply a cat-and-mouse game to see who can sneak up on whom. I was told that you are given a split second to try and counter an execution, but in the few times I got killed, I wasn't able to hit the mark.
If you happen to be a target (and everyone is, of course) and you get spotted, an alert will show up on your screen informing you that you're in sight. At that point, you must run through the crowded streets, and once you're out of sight, you can grab a hiding place by embedding yourself in a crowd, ducking into a straw pile, or planting yourself on a surreptitious bench. Stay hidden for long enough (about five seconds or so), and you'll get an "Escape," which awards almost as many points as a Kill would.
In practice, the action is simultaneously frantic and subtle -- when you're not running like mad from would-be killers, you're hunting your own quarry, trying to pick them out of the crowd. There's added strategy in abilities, with each player choosing two abilities like a sprint boost or a disguise function (which will make you look like a different model than you actually are), and those can mix up the gameplay quite a bit. And the game is balanced so that the better you do, the more you're challenged. If you take first place while the match is going on, you might have two or even three people hunting you all at the same time, with the chance for major rewards should you evade them all.
It's not perfect, however. While the premise is excellent, it's hard to see how the game will hold up over time. While the one match created some amazing stories (at one point, two enemies were on my tail and I broke their lines of sight at exactly the same time, sat down quickly on a bench, and hid in plain sight as they both ran right by me in different directions), it seems like the mechanic could lose its charm quickly.
And there are a few technical quirks as well. Most notably, because the execution move plays a quick animation, a few of the players in our match would experience the "kill queue" problem, where they could simply wait for an animation to finish, and then dive in to execute the victor.
If nothing else, Ubisoft can be commended for trying something new with multiplayer -- a deathmatch where actually killing your opponent is only the last of your interaction with the environment.