I can say that tanks rarely take fault -- at least not since Wrath -- and I regularly switch between roles. I've healed dungeons and have had DPS pull mobs, get 2-shotted, and blame me. Sorry, not even priests can dispel stupidity. If I can't heal you in gemmed and enchanted DS gear, pushing out 20k hps, odds are you're doing it wrong.
Also, insert obligatory "shoulda rolled a prot pally" remark here. I currently have three max level tanks (blood DK, prot warrior, prot pally) and have leveled a druid tank up to 74 in Wrath, and I could probably slam my head into the keyboard and hold an entire instance on my pally.
Blood DKs are satisfying because of the sheer amount of DPS they can put out while tanking. It's my spec of choice for soloing now because of their ludicrous self-heals and damage output (granted, I'm in half-DPS gear right now since I'm still collecting tank gear, but I can generally do 10-12k single target and 20k against trash with party buffs).