Didn't read all of the above but I'll give my take on it as I'm levelling a tank (27 Gladiator) for the first time now through dungeons.
There is high pressure on the tank (perhaps self inflicted) as there is more responsibility on your shoulders than playing any other class.
My job is to go in there and lead the way through the dungeon. This involves marking mobs along the way (working out which to pull and hold first and managing aggro on chain pulls).
Did Totorak earlier and a hotshot Bard decided to go whole hog DPS even though I told him I was still learning to tank...he thought kiting the bosses around while I madly threw out enmity skills like an idiot trying to get aggro back, was a good idea. I could easily heal or dps anything but there is certainly a lot more to think about with tanking I'm finding. Hoping it's just a phase that I'll overcome once I'm more confident but it's pretty obvious to me now why there's a tank shortage.