http://www.bethblog.com/2012/05/01/dawnguard/

Those aren't arrows on his back, they are crossbow bolts
Focus on vampirism, which was lackluster
Can hunt vampires or join a vampire clan
Dawnguard is a faction specialized in hunting vampires
Expansion set in a small area just off the gate to Morrowind, with Blacklight as a major town with an ebony mine
The moot is in effect and a High King is selected based on your actions during the civil war
Snow elves, goblins and cliffracers return as enemies
It's a timed exclusive, not only for Xbox 360, just like we did with New Vegas