The Watchmen Blu-ray will feature a normal and director's cut of the film, a splitscreen "Maximum Movie Mode" that shares the screen between Zack Snyder and the film, plus it...connects to Facebook?
In the interest of making Community Screenings a bit more practical (that BD Live function allowing users to watch a film in sync with friends over the network), Warner has teamed with Facebook in a cash-less deal to allow Community Screenings to sync with your Facebook friend lists.
It's actually a great idea that's far more progressive than using Warner's current solution, a proprietary account system. Read more about the DVD and Blu-ray at these links: [Hollywood Reporter and Rope of Silicon via BluRayFreak]
An additional note to Watchmen fans: The Director's Cut detailed here does not look to include interspersed moments from The Tales of the Black Freighter.