its so easy to overgear content, that people don't actually have time to learn how to play their role. Their gear provides too big a margin for error. Then when you get into something more challenging like HoR or even old kingdom (which is the toughest pre toc/icc instance, just because of shadowblast and flamestrike), they fall apart.
Old kingdom is trivial if you target the spellflingers and worshippers first and interrupt their casts. You can always tell a terrible/lazy tank when he's a dk or a warrior and a flamestrike or shadowblast goes off. If those two classes ever get hit by those and then blame the healer, they shouldn't be tanking. however, some healers also have interrupts, so you can do yourself a favor and mitigate the damage yourself.