As a note to druids thinking about rage vs shield problem.

Why are you worried? It's obvious that we will get the same rage x damage absorbed as warrior. When they change that they don't change something in warriors or druids formulas, they change something in rage formulas, applyied to both class. Every change from vanilla wow to rage has affected both classes, because when you are a bear,you are a warrior for the game mechanic. As already stated back at the begining of wotlk from a blue (i don't remember the exact post): every change in mechanic affecting warriors have it's counterpart for druids, that's all.

Shield-tanks can't block during stuns either, so there's no extra disparity between SD and block in that case.

I'll try out some math and comparison:

I'm currently at 32% crit, 50% avoidance, 36.5k armor, 6800 ap raid buffed, with haste+buffs I have an attack speed of 1.85 sec (I'm also hit capped and near expertise hard cap so I'll assume 0 misses)

Now we can easly assume an attack each GCD.

So I'll do 10+8 = 18 attack every 15 seconds or 1 attack every 0.83 seconds.

Now suppose the boss has an attack speed of 1.5 seconds, he will do 10 attack over 15 seconds. So on average I have 2 chance of proccing the shield every boss attack or 4 chance if we consider avoidance.

With a 32% crit, it's about 1-(1-0.32)^4 = 0.75 = 75% chance.

So on average I'll be shielded 3 attacks over 4.

Now with 6700 ap I'll be shielded for 0.25*6800 = 1700.

1700 damage absorbed with a 75% chance it's not bad at all.

For example our warrior MT has a 22% block chance with 1300 block value.

Those value are far below what will gain a druid.

Pro:
we will get a better uptime
we will get a better "block value"
we will get more magica mitigation

Contro:
We need to attack to get it (look at sarth 3d for example)
it will scale with raid buff more than the warriors/paladins so we will get a lesser effect in istances and 10 men.
Going from armor to block will make our damage more spiky.


Now let's look at a 2 target tanking:
Our attack/sec. will be the same but we can crit on both target. But basically we will take 1 attack every 0.75 seconds.
considering 50% avoidance, 1 attack every 1.5 seconds.
We can crit on both target so we have a (1-0.32)^2 = 50% chance to proc a shield for any attack. Considering that we will do on average a little less than 2 attack each attack taken we will have about the same uptime of 75%.

Now let's consider 6 target tanking:

We will do 6 attack each 1.5 seconds and 2 attacks each 1.85 seconds = 68 attacks every 15 seconds. Consider for example a swipe with 6 target we have a 90% chance to crit. Just make it 100% for simplicity. We will have our shield up every 1.5 seconds. But we will recive an attack every 0.25 seconds so we will have a 16% uptime. If we want to consider mauling for extra shiled the chance can go up a little to 20-25% maybe.

All that math (gross math I admit it but I only want to know the "magnitudo" of the problem) is to say:

The effective coolness of the new ability is based only on how much they will nerf our armor.
They will nerf our armor considering average damage taken.
It's true that more mobs will hit for less each than a boss so also with a lesser uptime we will cut of more % of damage for each attack but not for the total damage taken.

Differently from warr/Pala, our shield will lose effectivness becouse it will lose uptime scaling negativelly with the number of boss. So:

Will blizz balance us on boss fight? With what kind of pre-mitigation damage and attack speed?
Will blizz balance us on trashes or multiple add fights?

I'm under the impression that balancing us toward boss fight will make less able to tank adds.
I'm under the impression that balancing us toward adds will make us OP vs. boss fight.
Also I'm worried that we will lose our Hard-Hitting-Bosses primatum.

What is cool about those changes is that now every single stats on our gear has a defensive effect letting us scale more properly.

An idea regarding Savage Defense and multi-mob tanking: I was wondering if instead of putting a buff (stacking or otherwise) on the druid to reduce damage taken, how about getting a crit on a mob put a debuff on that mob that reduced the damage it did on its next attack?

I can think of two complications with that up front, in that the damage reduction should only be used against the druid that applied the debuff, and a question of whether an avoided attack would still eat the debuff off the mob.

On the plus side it would seem to match the mechanics of block a little more closely, where you mitigate damage against a particular mob based on a roll against that actual mob. The other models I've heard so far can have you crit on a boss, but waste your shield on an add, or vice versa.