It's mainly because of a lack of tank information out currently, all classes/specs. Healers know to a degree what they are expecting (loss of many instants, consolidation of abilities, Cata-like triage style healing though bliz said they will make sure mana isn't too hard to come by). DPS varies but given the pvp blog that was released we know there are going to be a reduction in burst across the board and many CDs are either going away or being consolidated, which can mean that on the pve side of things Ret could see a buff on sustained while taking a hit in burst. Time will tell.
There hasn't been a "Tank" watercooler or document dump. Without that information, even just a direction bliz is going with tanks, all we have to talk about is the lvl 100 talents, one of which is gone, bonus armor on non-armor slots and dodge/parry/hit/exp going away.
From the tank side of the game there is too much that we don't know, so we wait.