>>What is the untalented critical damage on a DK special ability? 150%, or 200% damage?
Runic Focus, the passive ability they introduced, means that their special ability crits are 200%
>> Do you get kill / quest credit while playing as a ghoul?
Yes. I died and Arazzius died while I was a ghoul and I got credit. Funny thing was, I didn't realize it. I asked my group, "how the heck did I live through that?" "Uhh... you didn't. Have you looked in the mirror lately?"
>> Is Blood Boil damage multiplied by the number of diseases? Does it remove the diseases?
Blood Boil does not remove diseases. I believe it's a dmg per disease, can't tell ya, it's not really in my rotation.
>> Does Pestilence refresh disease duration on secondary targets? (i.e. can it maintain diseases on all mobs indefinitely by using on a different target every 10s?)
You got it exactly! It will refresh the duration on everybody but your current target you just used it on. So with your 'ie' you got it exactly right.
Sorry I can't answer more. I'm just having a lot of fun with it so far and I'm still getting used to it all, haven't done any drilling down into anything yet. I can tell you which spec is more fun to play, not which one is necessarily "better". (that'd be frost, with blood being a distant ugly last place)