Well evidently I need to rethink my position because I just found a loophole. The "tank" was a DK with 5k gearscore and about 37k health, but I noticed before very long that he was taking huge hits. Especially on group pulls. Of course it didn't help matters that two of the DPS were also taking big hits -- especially the Mage who for some reason can simply not explain, wanted to run into the groups and aoe on top of them. In any event I noticed right away the tank was taking hits. If I hadn't had beacon on him (yes I was healing as Holy Paladin) he would have died quickly several times while I was healing the others. We did just fine on the Maiden and Krystallus, but we actually died on the Tribunal during phase 3. Despite spamming FoL on him which heals for 5-8k every 1.1 seconds he went down like a ton of bricks, and then everyone else did as well. The tank died again on Sjonnir.
Now keep in mind I am top healer in my ICC-10 group and I heal Heroics every day. I die of boredom healing as Holy in heroics, yet I let this tank die not once but twice. You tell me how that happens.
So what fooled me? I moused over him and saw he had a 5092 gearscore. After the first few pulls where I saw him taking big hits I inspected him and he had decent gear. Certainly nothing that screamed to me to drop group. Later on I went and looked at the armory but it showed him in Unholy spec as DPS, so I couldn't even look to see what his Defense stat was in tank mode (btw Blizzard, you need to change it so that we can actually see stats when we inspect someone). From the hits he was taking I can only speculate that he was not defense capped, and was low. Not just a bit shy. And I can only guess that he was letting things hit him in the back.
This folks is why tanks, prior to the new LFG system, stopped tanking for PUGs. And it's going to be why I stop healing in PUGs. So when you start to see a healer shortage you know why. Thank the "tanks" who have no business tanking because they don't know how and aren't ready.