"Vengeance: This is a mechanic to ensure that tank damage (and therefore threat) doesn't fall behind as damage-dealing classes improve their gear during the course of the expansion. All tanking specs will have Vengeance as their second talent tree passive bonus. Whenever a tank gets hit, Vengeance will give them a stacking attack power buff equal to 5% of the damage done, up to a maximum of 10% of the character's un-buffed health. For boss encounters, we expect that tanks will always have the attack power bonus equal to 10% of their health. The 5% and 10% bonuses assume 51 talent points have been put into the Protection tree. These values will be smaller at lower levels. Remember, you only get this bonus if you have spent the most talent points in the Protection tree, so you won't see Arms or Fury warriors running around with it. Vengeance will let us continue to make tank gear more or less the way we do today – there will be some damage-dealing stats, but mostly survival-oriented stats. Druids typically have more damage-dealing stats even on their tanking gear, so their Vengeance benefit may be smaller, but overall the goal is for all four tanks do about the same damage when tanking."
I've been thinking about this a little bit. This will mean that tank dps scales with HP - so 10 stamina also provides 1 attack power. I believe that 10 stamina also means 1 strength for paladins, so they'll get 1 strength and 1 attack power from 10 stamina. I don't really understand paladins, so I'll leave that for now. In addition you have vitality, which increases stamina by 9%.
I foresee a couple of outcomes
1) Blizzard's goal of making us care about things other than stamina will not be as successful as they would like
Blizzard have said that they want us to care about avoidance more in 4.0, by limiting healer mana and keeping avoidance down. Lots of tanks like that, and I guess people are intending to try socketing for avoidance a little more. Remember however that since stamina increases the effect of the vengeance buff, stamina stackers will also do more damage than avoidance tanks - not only because they have more rage, but also because will have more attack power.
Suppose I was choosing between a stamina trinket with 400 stamina, and a dodge trinket. I'd have to bear in mind that if I chose the stamina trinket I would have an extra (passive!) 436 attack power, in addition to the extra rage I would get from being hit more often. The potential attack power differential between an avoidance tank and a stamina tank could be very significant, depending upon how Blizzard tweak the numbers between now and Cata though.
2) Vengeance will be unbalanced
My unbuffed health has roughly doubled this expansion between stepping into Naxx and currently fighting Arthas. The differential is greater for 25 man raiders, but remember that without the extra tiers in gear required to differentiate 10 man and 25 man raiders the gear curve is likely to be flatter in 4.0.
Dps numbers however have increased by a factor of about 5 or 6 - whereas 2k was good in blue gear in Naxx, now the top dpsers are pulling 10k plus. On the assumption that a flatter Cataclysm gear curve will compress this to say 4 times, you still have dps increasing twice as fast as the vengeance buff.
This implies that either vengeance will be massively overpowered early on, or it will be too weak later on to keep up with runaway dps. I'm not saying that we'll have threat problems in tier 13, because we don't know how it will work in practice, I'm simply saying that the rate at which the vengeance buff increases will probably not be as great as the rate at which dps increases.