For 25man, I'd recommend at least 30k health on your MT and at LEAST 18k health on every other raid member (for the vortex aoe damage). Anything less is going to really stress your healers.
For 10man, it's a little less demanding on gear requirements (I tanked Maly with 27k not long after WLK's release, but he had some bangin' healers), but the fight is exactly the same. Raid damage is a serious consideration and if you don't have an exceptional raid healer you're going to rack up the repair bills on this one.

