Here is some math on why, as far as I can calculate, savage defense will be a big nerf..
Assume raidbuffed, you have 6000 AP and 40% crit. This is not too bad numbers anyways. Yours may vary, but you can always redo my calculations with your own numbers. Just because I've calculated my own armor mitigation numbers, I'll use mine. These are 34928 raid buffed in 3.0.9 and will become 29013 in 3.1.0 if they go ahead with nerfing SotF from 66% to 33%.
Mitigation factor with PotP and my armor in 3.0.9 = 0.323 * 0.88 = 0.28424
Mitigation factor with PotP and my armor in 3.1.0 = 0.364 * 0.88 = 0.32032
Extra damage taken in 3.1.0 due to lower mitigation: (0.32032 / 0.28424 - 1) * 100% = 12.69 %
Since Savage Defense don't scale with incoming damage, we need to know boss DPS and attack speed in order to calculate how much damage we mitigate with the shield. I ignore avoidance, and calculate as if all attacks hit. My calculations follow:
Savage Defense Factor = (Boss damage per attack - Average damage blocked by Savage Defense) / Boss damage per attack
Boss damage per attack = Boss DPS * Boss attack speed
Average damage blocked by Savage Defense = Damage blocked by Savage Defense * Chance of critting between boss attacks
Damage blocked by Savage Defense = Attack Power / (4 * Mitigation factor)
Chance of critting between boss attacks = 1 - (1 - Crit chance) ^ Average number of hits between boss attacks
Average number of hits between boss attacks = Boss attack speed / Druid auto attack speed + Boss attack speed / Global cooldown
Here are the average damage mitigated by savage defense been calculated from various test boss damage scenarios:
Boss DPS 70000, 1s attack speed: 2.81 % reduced damage (Patchwerk)
Boss DPS 40000, 2s attack speed: 4.27 %
Boss DPS 20000, 2s attack speed: 8.53 %
Boss DPS 10000, 2s attack speed: 17.1 %
And retried with 100% chance of shield being up:
Boss DPS 70000, 1s attack speed: 6.68 % (Patchwerk)
Boss DPS 40000, 2s attack speed: 5.85 %
Boss DPS 20000, 2s attack speed: 11.69 %
Boss DPS 10000, 2s attack speed: 23.39 %
We need a boss with less than 20000 DPS for this to not be a nerf (More than the 12.69% mitigation lost in armor). Even if we assume the shield is always up. Isn't that an easy encounter anyways? Doesn't sound like an encounter where healing is an issue. On top of this, not only is the shield crap unless you're taking very little damage, but it's unreliable avoidance instead of reliable mitigation too.
According to my calculations, based on a TTL rating of stats (
World of Warcraft - Evaluating druid tank gear), even, assuming 10000 boss DPS, 1 AGI is worth more than 10 AP or 5 crit rating with my current gear level. At patchwerk levels, 1 AGI is like 75 AP or 25 crit rating with my current (but not awesome) gear level.
This makes AP and crit really bad stats for survival purposes, compared to their item budget values.