I just thought I'd add I just cleared a few heroics last night with disc priest, mage x3, panzerkin (moonkin tanking ftw)

Moonkin has about 19k health and is not crit capped but didn't have any problems tanking old school 80 heroics like violet hold and does way more DPS than my much better geared real tanks do.

Kind of a fun way to use an underpowered caster class once you outgear content sufficiently.