You get to kill the leaders of the Dawnguard..turn your wife into a Vampire (I turned Lydia, couldn't resist) and create three fledgling vampires in several different cities (only 3 cuz Vingalmo glitches out after then and wants you to turn the same person after they are already turned) and you can get ancient vamp body parts to keep extending the blood of the ancients effect to a max of 12 days.
Couple other quests to impersonate a Dawnguard and kill random members of the Dawnguard too, always fun if you want enchanted crossbows and bolts.
Game randomly selects a radiant quest for you after you turn in the one you are already doing so if you want to be safe that you won't get a Vingalmo glitch save before turning the one you already have in.
Game doesn't randomly select them when you ask how can you help, only after you turn in the radiant quest so saving after turning it in and before asking how can you help always ends with you doing the same radiant quest.
Oh and you also get two random quests to get two sets of artifacts.
The two blood magic ones that give you +100 HP and +20 claw damage and one that gives you +100 magicka and magicka regen (I <3 the last one) and two night magic ones that gives you the bat perk without actually having the perk and the ability to summon two gargoyles..three if you have that conjuration perk.
Yeah, I already did all those and got those magic items, then afterward I went on to doing the "carbon copy errands" of turning people, killing Dawnguard, etc. Funny thing is, the very next time after someone mentioned in this thread that I would run into a glitched quest, wouldn't you know it: I got to the one where I was sent to turn someone I'd already turned. That basically marked the end of my tenure with the Volkihars, and so I moved on to finish up other things. And by the way, if anyone is stuck trying to get back to being a werewolf again, all ya gotta do is go to Aela in Whiterun in Jorvasskar and have her do it for you, if you don't feel like bothering with getting Falion to cure you. The result is all the same; there's nothing really special about what Falion does--just a short ritual and you're cured, end of quest. If I'm not mistaken, I recall someone having an issue with that, so I hope that if they haven't solved it yet, then they might find hope here...
Now I have another big question: Legendary Dragon. When and where should I expect an encounter with the Big Bad? I saw an earlier post that said you have to be around level 70 or so, and I'm at 68 right now, so...anyone...?
P.S. You can enchant those crossbows yourself if they're blank slates, just like any other weapons, and you can improve them on a grinding stone as well. The damage they do will be commensurate with your archery skill, as will be all other attributes, which is cool considering that if you have the "quicker draw" perk in your archery tree, that applies to faster loading on the crossbow too.