Careful there, only the overflow block from Shield Block is converted to crit block, not all overflow block.

So it looks like Shield Block increases Block Chance AND Crit Block Chance by 25% AND applies the overflow to Crit Block Chance.

Mini Shield Wall not so Mini anymore.

An examination of the effects of Shield Block and Critical Block at Wowhead appears to indicate that they're using the same aura applications for both shield block and mastery. It is possible the 25% Critblock is because despite what the tooltip says, the actual effect of shield block is to give you 25% worth of mastery, and after that, convert excess block to critblock.

Pretty significant oversight on my part... but moving forward then:

179.28 mastery rating would bring you 1.5% critical block.
176.71899 dodge would bring you 1% pre dr avoid.

1% pre DR avoid is 1% damage reduction all the time (independent of shield block - pre DR)
1.5% critical block is 0.642% damage reduction (before shield block)

So when does avoidance DR to under 64% efficiency?
~2500 of an avoidance rating.

averaging in shield block, requires closer to 3000 of an avoidance rating to balance.

So with this strange shield block behavior and assuming we are getting 25% mystery bonus on the shield block rollover calculation, I have reran the numbers to find a point where you reach 100% critical block with shield block up.

The magic number seems to be somewhere around 30 total mastery (22 from gear). This changes a little with avoidance stats. For arguments sake, let's assume you have about 25% from dodge and parry which seems to be the norm with current gear.

5% miss + ~25% total avoidance from dodge and parry combined (9% dodge and 16% parry) + 5% base block + 15% sentinel + 12% base mastery + 33% block from mastery gear + 25% shield block + 25% mystery bonus = 145% - a 45% rollover to crit block.

Crit block would then be:
45% rollover + 33% crit block from mastery gear + 12% crit block from base mastery + 10% from hold the line = 100% crit block

That is approximately 3961 mastery to have 100% crit block with shield block.

Something tells me this isn't the intended behavior of shield block. I anticipate a nerf.

We had noticed this really early in our raiding 10 mans about the 25% extra critical block chance. It definitely does not seem to be intended and in our eyes it has put the prot warrior significantly ahead of all other tanks in mitigation right now with paladins following in second place. Whenever they get around to rebalancing the tanks (poor feral druids) they will probably either catch this glitch or adjust something else to compensate for it.

I'm enjoying it while it lasts though, I know this is a place of numbers and not hunches but my shield block feels more powerful than shield wall in many situations at the moment. Higher mastery just seems to amplify this effect. This should not effect Mastery from being the best avoidance stat to stack though, but it will diminish the effect of it a bit.

I don't think the meta choice is necessarily dependant on which one gives the higher overal damage reduction. As was shown by the math, the difference is not huge.
If you can guarantee that in the fight you will be able to block all the time, then the block meta is slightly better. This is not always the case though.
I will argue that decreasing the damage taken from already blocked or critically blocked attacks (which is already pretty weak) a bit further due to higher block value, is often less important than reducing the incoming damage from unblocked attacks through armor (be it due to stun, movement, or simply not having reached sufficient evasion levels).

From what I have seen in raids so far in Cataclysm, healers seem to have more problems with heavy bursts that require them to spend more expensive spells than with slightly higher damage intake over time.

Very interesting thread and the Shield Block situation really is confusing. Has anyone tested if the numbers are correct or if it's just a tool-tip error from Blizzards part?

Also, according to the single-target rotation you've completely left Shockwave and Concussion Blow out. As far as I can tell they generate more DPS and DPS / Rage than Devastate and esp. if considering the GCD prior to Shield Slam off cool-down they should be more efficient to use, shouldn't they? A Sword and Board proc here would be more rage-efficient and perhaps would lead to 1 more heroic strike and / or the use of Inner Rage, but if you're not rage-starved, wouldn't the use of Shockwave / Concussion Blow be better?

Also the application of rend by using Thunder Struck from Thunder Clap... I presume we want to use this the lats 2 or so seconds of the Rend application? Or is it better to let rend tick off to 0 and then reapply it using rend? (again, I'm talking single target).

Seeing as how most tanks are gemming pure mastery, I don't think your argument is too valid. Don't get me wrong, I much prefer the 5% block metagem, and it is the one I am using, however, most tanks are already using 20mastery/30stam in blue sockets, 40mastery in yellow sockets and 20parry/20mastery in red sockets. As long as you have 150k unbuffed health, avoidance will help out more than a larger health pool (which requires more heals to top off)

Anybody know of a log analyzer that identifies critical blocks? I haven't found one yet so I did some manual analysis using excel to try to confirm the stats. I did not collect enough data for it to be scientific yet - just used a sample to set up some macros to be able to calculate the results.. I'll try to focus on it tonight.

My gear is crap though. I could benefit from a log file from a properly geared tank to run the analysis on.

Should SB damage reduction be over 20%? It's only a 30s CD.