BioWare has shown off the soldier and rogue class types, but today they unveiled what is sure to be one of the most popular classes in The Old Republic: the Sith Warrior. The page details some favorite Sith hobbies, such as channeling destructive feelings, wiping out the Jedi, and stamp collecting. One of those is a joke; see if you can spot which one.
It also mentions a “Dark Council” that may serve as structure for the Sith missions, and of course most conflicts will be against the Republic. Obviously the Sith don’t work particularly well with others, but the description does mention that they can ally with Sith, Bounty Hunters, “or others who embrace the darkness.”