EDIT:: Are you talking about Dawnguard, or jut the update???
This WAS in the Dawnguard forum when I replied.This is all in Dawnguard only........Lots of stuff...
Dawnguard obviously has a new big quest line, with a vampire overhaul, and new weapons/armor/spells to find. etc
Crossbows (and they can be upgraded to more powerful versions if you side with the Dawnguard)
New tougher Falmer enemies, wearing new Falmer heavy armor (you can wear it too)
New type of chaurus enemies (they fly!)
Dragonbone weapons can be crafted. (all the standard types of weapons)
Arrows of all types can be crafted.
A few new shouts to learn.
Two new stronger dragon types, not seen until you're very high level.
A large new Side Quest, not associated with the Dawnguard content (and it's quite good!)
there's likely more, I cannot think of right now