As far as i can tell bud you can run around i finnished the dark brotherhood and it just lets you roam i may play it all again as i missed1 achievement when i killed the bloody dog!!! and yeh i fancy doing a magic dude for a change as i was maxed on 1 handed block 2 handed and archery so might do... problem is i now have assasins creed to start 
You'll complete AC:2 - Revalations really quickly, every spin off of AC:2 seems shorter and shorter. Only thing that consumes anytime is the capturing of Den's and training assassins, main story is very short, but some missions are quite cinematic like when the arsenal is on fire, or when you are chasing a boat with templars. But it feels like a really short game compared to the rest so far.
Werewolf thing is something to do with the champions, the vampire thing is simpler, when a vampire uses a drain spell there's a 10% chance of getting a disease, if this disease isn't cured in 4 days you become a vampire. Some pro's and cons to a vampire and same thing with werewolf:
http://www.uesp.net/wiki/Skyrim:VampirismAlso whats the pro and cons of destroying the dark brotherhood?