There's a lot going on in the latest Assassin's Creed: Brotherhood developer diary. It covers everything from the Templars to Desmond's mission to new VR training exercises. Anyway, we're sure you don't need us to give you excuses to watch it. In fact, you're already watching it aren't you?
Rather than using money earned to bolster a villa, you'll fix up Rome itself, which will change Ezio's social standing, unlock missions and curry favor with various factions. The release promises collaboration with Da Vinci, Machiavelli, and Caterina Sforza, and describes new "secret weapons." Ubi is throwing in poison darts, parachutes, hidden blades and guns, not to mention an "advanced flying machine" -- which is presumably useful in more than a single mission this time.
Multiplayer -- developed by Ubisoft Annecy, who created the online component of Splinter Cell: Chaos Theory -- is said to feature "multiple authentic character classes" (presumably the characters seen in the gallery below) each of whom will use "signature weapons and killing moves."