I'm several hours in, so for those of you stuck at work who are dying for spoilers but can't play yet, here's some info:
* New smithing patterns abound. Not only can you now craft dragonbone weapons, you can now craft arrows of every type, and there are new types of jewelry added (Bone Hawk Amulets - no listed special effects, but I haven't crafted one yet myself), and a Misc. column added with a new plan called 'shellbug helm'. Looks Falmer-ish.
* Early on in the dawnguard questline you have access to vendors that sell Dawnguard armor. The full set increases the damage you deal to vampires, and the dawnguard shield does additional damage to vampires while bashing.
*New Spell: Sunfire. It's a restoration spell that does 25 fire damage to undead. Available from Dawnguard vendors early on.
*You get a crossbow (basic) in the first mission, before you get to choose dawnguard or vampires. However, once you choose dawnguard, you're treated to additional sidequests of the radiant variety, in which you locate dwemer schematics for enhancing the crossbow. Enhancements include additional armor penetration and exploding bolts.
*No matter which path you choose, you can obtain Seranna as a follower, and not only does her character model look excellent, she's quite possibly (IMHO) the best example of quality voiceover work the Elder Scrolls series, with the only exception being Sheogorath (which, let's face it, nothing can beat that).