@Wellsee
"older content - like heroics once we outgear them." I think what you might be missing here is that Mobs die so fast once you outgear the 5 mans that you truly only need to hold the mob for 10 seconds or less anyway. As a Warrior Tank I get initial aggro and focus my threat on the melee mobs in the group (since they generally kill DPSers once they grab aggro), if I lose aggro on something long before its going to die then i will just stun the mobs and by the end of the stun they are generally dead. I think it is a very far stretch to think that tanking older content will be hard in any fashion.

@Sluetel
If you are having trouble with initial aggro then your raid buddies are most likely popping all cool downs right off the bat and your rogue or hunter buddies are not starting the fight off with tricks or misdirect as they should be. Or you have some of the highest DPS friends around. If you are talking about 5 man threat on the other hand then the DPS need to wait until you at least get some kind of an attack on the group of mobs, as prox aggro obviously is not going to hold aggro. I am of the sound mind that if at the start of a fight if I see an attack fly past me while I am trying to get to the mobs that person can tank the guy he hits. At that point it is between them and the healer.

@Everyone
This mechanic is fantastic and even if they need to tweak it here and there as we go this mechanic will balance threat issues in the game.

For those concerned about how it will be over powered in PVP you need to think about how it works. The tank must be taking consistant damage for it to stack and not fall off, and I know that when I am taking damage on my tank I die long before Vengeance stacks up to make a difference, if I am not getting heals. If the tank is getting enough Vengeance in PVP to start doing enough damage to cause you problems then you need to go kill the healer allowing this to happen.

Go GC!