The coolest feature seems to be the ability to turn into a Vampire Lord, which you can do if you're a vampire by hitting the R1 or R2 buttons on your controller.
The Vampire Lord can float, but can't properly fly, which is a pity. The Vampire Lord has many cool moves such as the ability to summon a gargoyle pet to help you attack. He also has the ability to turn into a cloud of bats and do a short-distance teleportation. This can be done twice before the ability has to cool-down.
The Vampire Lord has his own talent tree, which you can access by hitting B. The top-tier ability is constantly having a cloud of bats surrounding you which will attack anyone nearby.
There's also new weaponry such as the crossbow. Once equipped you can do a melee attack with the left trigger and a crossbow shot with the right trigger.
Developers are estimating that there will be approximately 20-30 hours of gameplay and additional side-quests in this pack.
Dawnguard will be released in Summer of 2012 on the Xbox 360, PS3, followed by the PC version. Beta for this pack will start soon.
