I have experience tanking through UBRS (it deserves this honorable mention), Zul'Gurub and Molten Core (never did quite down Ragnaros, however) in Vanilla WoW. With BC came the realization that I was effectively marooned on a server I didn't hold as much love for as I once had. So, I rerolled - Horde, interestingly enough, as a shadow-then-holy troll priest. With him, I made as far as Serpentshrine Cavern and Tempest Keep, though I never did down any bosses in either instance. I'd like to blame the guild, but that seems a little too audacious. It may behoove me to note that I regularly topped the healing meters and tended to be above par in certain areas. Most memorably was The Lurker Below boss fight in which I was complimented a number of times for my, for lack of a better term, competence (as I expected at least half as much from my fellows). I was perhaps the only person to regularly avoid spouts and whatever various water-pumping means Lurker possessed, while simultaneously not aggravating healers with a ridiculous amount of unnecessary time in the boiling water.