Despite how much people say the DLC svcks and: what it should have, what ruins it, what makes them mad, what bugs it has, what doesn't work, that it is too short, that it is too costly, that it has missing items, that it has broken side quests, that it has missing dialogue, that it has uncooperative npcs, that it has broken magic, and that it has open ended plots.

We can now:
-Change our appearance for 20$!

- craft Dragonbone weapons.

vamp lord
werewolf perks
forgotten vale
fort dawnguard/castle volkihar
red skooma
new ways to feed
gelebor

* 2 Faction Questlines

* Vampire Lord

* Vampires look more... like a vampire

* Wild Werewolves

* A lot more random encounters

* A good side quest (Lost to the Ages) with a good reward

* Tons of sidequests

* The Forgotten Vale

* A very cool fight in The Forgotten Vale

* New items

* New books

* New spells

* New dungeons (darker ones too)

* New enemies

* More content within a fascinating world

Vampire Armor...most awesome attire in Skyrim that seems to have been made just for me...

I really liked the questline; particulary from the vampires' perspective. Filling the Bloodstone Chalice was a great quest, with a nice sidestory on the Redwater Den. Climbing the castle's tower in search of Valerica was amazing, it really gave me a Castlevania-esque feel.
The radiant quests have also been much improved, now providing more context and motivation for your tasks. I actually enjoyed doing them this time.

Jiub.

Serana is the best A.I. companion in the game (she comments on places you visit, she keeps herself busy when you're idle, she dual wields staves in combat). The vampire armor looks great & the Dawnguard armor can make your character look like a 12th century crusader. And despite the numerous bugs the two questlines are lots of fun. Overall I think it was $20 dollars well spent. : )
And I second what Crafty_Biscuit said about making your way up the tower... that part is a highlight for me.

Unless you're a vampire...

Zephr
Bone Hawk Ring
Ancient Falmer Armor
Not much of a fan but Crossbows
Savage Feeding Perk(Nearly infinite werewolf transformation)
Dragon Bone Weapons(Finaly a rival sword for chillrend)

The Forgotten Vale and Aethrium related quests were pretty darn cool.

Being able to change your look was pretty darn cool - honestly, setting this in the first place is a rather hurried process.

Also, the recurring/radiant quests are less boring than the regular ones (e.g., Companions, Dark Brotherhood, Thieves Guild, etc.).