I DLed this the other day since I wanted more Skyrim stuff. I tried using one of my two characters that have completed the main quest. Here is my take on some of the dialogue of Dawnguard with a minor spoiler warning

Chick in box- Wow how long have I been sleeping?

Me- A long time, say is that an elder scroll on your back?

Chick in box- Yes it is a very rare and powerful artifact which few may ever possess.

Me- Well what do you know I have one too, what a coincidence!

Chick in box- WTF!! does everyone in this era have an Elder Scroll!??!

Me- Wellll, not everyone...

Then we meet head vamp...

Head Vamp- "I can give you the power of a Vampire Lord blah blah blah..."

Me- "Well gee, I don't know you see, I am already a werewolf and can summon dead heroes of Sovengard, and also have this pet dragon that comes on command."

Head Vamp- "Get out!"

Other than that I find using a character that has done the main quest is kind of boring(aside from nobody seeming to know who you are). I tried a higher difficulty but that was not much better. So now I am trying to play dead is dead on regular difficulty, so far got killed at level 7 by going somewhere I should not have. Still much more interesting if you play for keeps.