My experience has been that it becomes increasingly more difficult to find good tanks in general. In the past, it was healers one tended to have the greatest trouble PuGing. Can't speak for all servers, but in generaling, when going through LFG, finding a tanking class period has become fewer and further between, and from there....
You get the guy like the one I mentioned who argued "I'm fury spec, I can't learn sunder". And finally after a bit of arguing with the group, I finally come out and mention that my fury/proc spec warrior (really only 10 points in proc now) has had sunder armor for ages.
Until someone I knew, who used to MT for my former guild got a healer from his kara team, and mage from his guild to help me finish BM, it's also what I ran into trying to PuG BM. After like 4, 6, or how many other days, tryign to find a healer and a tank so I could speak with Medivah at the end, and I mean like 9 hours a day, I along the way finally did find a couple groups, that didn't go.
In one of them, the healer and the tank were in a shouting match with each other. The tank accused the healer of not healing him enough, and the healer was like "what can I do, my heals are nerfed by 50% here, and I'm out of mana". This was at the second boss. A shadow priest in the group (not the healer), PSTed me, and was like "there's no way this tank is geared for this instance. No one person could heal enough to make up for the damage being done". But he left it at that, and didn't want to say anything wrt those 2's shouting match, publically.
There's a couple good tanks I know of in general, but also rather busy. One of them was in SMV, when I hit it, so helped me get Ruul the darker for this quest chain I'm on. However, without a healer then, it's hard to get onto the next part of the chain (think it's going to have to be organized)
I'll think about the 3 challeges of the sha'tar (which another guild, actually healer is on), when I get past this one. And again, the same problem facing her, as with me, finding a tank... And of course (beyond the cipher of damnation), one's talking heroics.