A friend and I duo-ed Karazhan and AQ20 with a Resto Druid and a Blood-spec Death Knight respectively.
Karazhan is interesting because most of it is easy enough to solo. The only bosses that would fuck up any solo-ers looking for a full clear is the optional boss Nethersprite, mostly because the mechanics; and probably the Opera Boss, depending on which one you get. (Romulo and Julianne need to be killed within 10 seconds of each other or they're rez each other; Big Bad Wolf will turn you into a helpless Little Red Riding Hood for 20 secs or so; and the Wizard of Oz one has six bosses, five of which you may have to fight at once)
Nethersprite is actually quite easy if you make sure to stand between him and the green beam to stop him from healing back up, but because you can't block the beam after a certain amount of time after you've been in it, you definitely need at least one other person to help you. Or you have to have a shit ton of DPS.
Interestingly, while physical attacks won't work on Nethersprite when he phases, I was finding my DK's magical abilities, like Icy Touch, Outbreak, Death And Decay, Blood Boil and Death Coil still worked just fine. In fact, I ended him with a Death Coil while he was phased.
Oddly enough, the end boss of Karazhan is actually safer being taken on solo since he has a nasty little ability to reduce health to 1 and prevent healing for a few seconds, and he'll toss this on up to 5 different people in one go.... but he will never use this ability on the main tank for obvious reasons. (i.e. even Blizzard aren't THAT big of dicks).
BTW, I feel like a fucking idiot for not noticing the massive change to Death Knight's Raise Ally ability. As of 4.1.0, Raise Ally is now a battle rez for Death Knights. I didn't realize this until we did Karazhan.
Also, we couldn't find the urn that lets you summon the second optional boss, Nightbane, so we had to leave that one unfinished, but we cleared the rest of it, including the chess match (only took us two tries). Is the urn spawn bugged or something? Anyone know? Or does it disappear if you kill the end boss before tracking him down?
In any case, afterwards, we did AQ20.... the only real boss that frustrated the SHIT out of me is Ayamiss the Hunter.
Why?
Because the boss flies high for Phase 1 and one can only hurt it with distance attacks.
Death Knights don't have much in terms of ranged attacks, and I had to jump in order to be able to hit the boss with Death Coil, Outbreak or Icy Touch. And nothing else I had seemed to hit it at all in Phase 1.
And I have no idea how, but my friend was quickly swarmed and killed when he tried adding his DPS to killing the boss, and again after I used my battle rez on him, leaving me to solo this EXTREMELY aggravating boss.