I think around 45k health, with say, 88%+ mit and a better than 1:8 hp:ehp ratio (I have fairly relaxed standards b/c I understand we would all like to pursue dps as well), it should be good to go. At the end of the day, there will be some cases that nobody in the world can tank even if you stacked hp or even if you stacked to 92% mitigation, you can still die. Rather than worrying about corner cases (here's a dozen pages of a few guys obsessed with boosting mitigation to basically not even die:
http://us.battle.net/d3/en/forum/topic/8569386969?page=1), I think we can just move the mouse a tiny bit and sidestep stacked molten, plagued, arcane turrets, etc. (I honestly don't worry about molten explosions, mallet lords, uber leorics - any of these in of itself should be no problem, it's when 3 molten explosions go off together, or leoric walks, swings and magda hits you with some flies at the same time that you could die, c'est la vie eh?). For all of these cases there's really one good answer - UA. That about takes care of it ^^