Positioning is a problem. and the reason for that problem is that stupid mob movement. When TC is on cd or mobs are stun immune, you run around and do circles like if you were on drugs. You can hardly avoid hits in the back. And if then a boss decides to make a cone attack (3rd boss shadowfang for example), the drama begins.
Threat is no prob at all. Marking skull for 1st tgt and you don't even have to cc (except of some cases of course)
Using surv cds at the right time is not a problem. If you tried 1 or 2 times an encounter, you will know when its time for o-shit-buttons.
Positioning and mob movement is an issue. But i don't whine about, i have to handle it and thatsfore i am tank.
As far as i experienced cata heroics so far, saving healer's mana is the real challenge. Interrupt, whatever is interruptable and avoid the void-zones.