When I first dinged 80 I was just a tad shy of 540 Def. It wasn't because the gear was hard to come by, I just had my head down levelling and didn't take the time to plan. I got by early with the lesser flasks of toughness for a couple of runs until I did my homework and got the gear.
Most classes have a stat (or two) they have to hit some minimums or they don't perform well. Tanks are the same way. The difference is the price for poor performance with a DPS or healer is that someone else has to pick up your slack but a tank brings about a wipe. I am ok with that.
I think gambling with the raid group against a RNG is irresponsible. It may never happen but when it does you are 100% at fault. You can't expect a healer to cover for that. That is like blaming the healers for not healing you through Sarth 3D just because you can survive a single hit but not 2 in succession.
I am currently tweaking my gear for Sarth 3D and am just a couple points shy of 540 but you can believe that when we pull Sarth, I'll have 540 minimum.
Ideus, Blood Elf Death Knight
Officer, Tears of the Phoenix, Ysera US