As the title suggests, Ezio (our hero from ACII) is building a brotherhood of merry assassins in this sequel. They can be summoned to instantly take a guard or two out or they can fight alongside you if you're feeling overwhelmed. Members of your brotherhood can be trained by sending them off on their own to complete missions that you'll never see. Think Final Fantasy Tactics (I had a good feeling!). You communicate with members of your secret organization via carrier pigeons. Find a pigeon coop and you can enter the brotherhood menu and manage your team.
A major activity of Assassin's Creed: Brotherhood is toppling Borgia towers. The corrupt Borgia family has seized control of Rome and in order to give the city back to the people you'll need to burn down the guard towers keeping watch. When you locate a Borgia tower you'll need to infiltrate it, locate and kill the leader, and then scale to the top and set it on fire. This will clean the Borgias out of that zone of Rome and local merchants will start to return and set up shop.
In your travels around Rome you'll spot Borgia flags that have been planted in hard-to-see and sometimes hard-to-reach places. This is another new activity in Brotherhood. For every flag you find and remove you'll weaken the Borgias' grasp on the city.
If you played Assassin's Creed II you may have grown tired with looting bodies for cash. In Brotherhood, every fallen enemy is a store. Now you'll find useful stuff like medicine and crossbow arrows on each body (yes, you have a crossbow now).
And finally, Ezio has a cool mini-hang glider he can use to break long falls. The Ubisoft developer showing us the game didn't go into detail about this feature but did use it so we can see how it works.
Assassin's Creed: Brotherhood is out November 16. This ACII fan is excited.