Changes to mob damage are still ongoing, and it's something we're hand-tuning on a dungeon-by-dungeon and boss-by-boss basis.
One issue that has likely been significantly impacting the feel of tanking in dungeons and in multi-mob scenarios, is some legacy logic from 5.4 Vengeance that is no longer needed or applicable in the current tanking environment.
On live in Mists, Vengeance has diminishing returns with respect to damage taken from multiple targets, because such damage proved to be much less deadly in practice than the same amount from a single source. See this post for details: http://us.battle.net/wow/en/forum/topic/9573038382#12
However, with the general increase in tank health with respect to incoming damage, and the move away from "spiky" damage intake, tanks spend much more time partly injured. In Mists, what killed tanks was sudden bursts of damage. In Warlords, what kills tanks is sustained damage that exceeds incoming healing and self-healing. And when it comes to sustained damage, it is immaterial whether you're being attacked by one huge enemy or many smaller enemies.
Thus, we're removing any diminishing returns from Resolve when tanking multiple targets. A hotfix that makes this change should be in effect on beta within the next day. This should significantly improve survivability in dungeons and other multi-mob situations.