It was pretty surprising when Ubisoft announced that they were bringing multiplayer to the Assassin's Creed series with Assassin's Creed Brotherhood; all of the games so far have been completely solo experiences, and it's worked out pretty well for them, with Assassin's Creed II in particular garnering plenty of critical acclaim. At E3, the publisher managed to convince most of the people on the floor that the multiplayer would make sense and be a lot of fun, assuaging fears of tacked-on gameplay that would detract from the single-player experience.
This trailer, fresh from San Diego Comic Con, should give you an idea of how the multiplayer in Brotherhood will work, with assassins working together to take down common foes. I already thought the game looked promising based on what I saw at E3, and I can dig the idea of running around in costumes with a few friends, trying to rack up some stealthy but bloody kills. I call the harlequin costume.