If you’ve spent much time on this forum in the last several months, you’d have noticed a common theme is that tanks start to have threat problems again at extreme levels of gear. The problem really isn’t that surprising. Say that tanks start out doing half the damage of DPS specs. All is well. But the DPS specs continue to improve their DPS stats while tanks continue to improve their survival stats. Even if tanks spend a little effort on threat stats (some of which they get naturally on their gear), they still can’t keep up with the DPS specs. It’s a gear scaling problem.
We considered, and rejected, many other solutions to the problem, such as increasing threat modifier or choosing to no longer make tanking gear. Ultimately we decided that there were good things about the way rage works on warriors and bears (translating incoming damage into threat) and the way the mage talent, Incanter’s Absorption turned damage taken into damage done. It just provides the damage increase in a way that’s controllable.
Vengeance is NOT there so that you no longer have to ever worry about threat. It’s fine with us if you have to consider threat a little bit at the start of a fight. Again, if we wanted to make threat not a factor in WoW, we’d just remove it and have mobs always stick to you rather than just cranking the threat numbers up so high that you don’t have to take it very seriously.
Vengeance is also not designed to keep tank dps high no matter what in any circumstances. It’s designed so that when you’re being hit, your damage stays elevated. The damage done scales with your health, essentially allowing tank DPS to increase as DPS specs DPS increases. It scales a little bit with damage being taken too so that you don’t turn into a juggernaut if a rogue sticks you with a dagger in PvP. (Source)