This is an exceptionally long post filled with math. There is no TL:DR version.
Introduction: Everyone who has looked at warrior tank mechanics in the Beta knows that they have Shield Block, and Shield Barrier. It has been said many times that Shield Block will be better on harder-hitting bosses, and Shield Barrier on weaker. So what is the dividing line? When is, in fact, Shield Block better? How hard is "hard-hitting" in this context?
I started with some warrior stats similar to mine in the Beta. Yes, my warrior is still 85, and for this comparison, that's good, because I can compare my results to currently existing content that has been in the game for months. This means I'm using a dodge/parry/miss combined chance of 40%, a block chance of 30%, a critical block chance of 70%, and no unusual spells, effects, or items. I am assuming there is no clipping of durations of either Shield Block or Shield Barrier. I'm also using a boss swing timer of 1.5 seconds and a maximum Shield Barrier value of 40,000 absorption.
Now: Shield BLOCK can be used twice per 15 seconds, and it lists having a duration of six seconds. That means, at the maximum, you can have Shield BLOCK active for 12 out of every 15 seconds. I will now call that "Max" Shield Block uptime. I will also designate a "Standard" Shield Block uptime of 6 seconds out of every 15. To keep things fair, I will also assume the same amount of uptime/usage for Shield BARRIER, i.e. either two uses of Shield Barrier at "Max" or one use at "Standard" settings, per 15 seconds.
Block is now on a "second roll" system which makes it happen a lot less often than Live, especially when combined with the much lower Block chances on the Beta as well. If this warrior with these stats were attacked 10,000 times, then of those 10k times, 4000 of them would miss, be dodged, or be parried. Of the remaining 6000 attacks, 30% of them, or 1800, would be Blocked, and of those 1800, 70% of them, or 1260, would be Critically Blocked. The remaining 4200 attacks would be normal hits. If each attack did D damage on a full-strength hit, this becomes
(4000x0)+(1260x0.40D)+(540x0.70D)+(4200xD) / 10,000 = 0.5082 D
Without using Shield Block, each swing does, on average, 50.82% damage.
Now if you then use Shield Block, during its duration, every single attack that does not miss entirely becomes Blocked. 4000 attacks still miss, but of the remaining 6000 attacks, 70%, or 4200, are Critical Blocks, and the other 1800 are Blocks. This means
(4000x0)+(4200x0.40D)+(1800x0.70D) / 10,000 = 0.2940 D
While Shield Block is up, the damage per swing averages to 29.40% of its normal "hit" value.
If we went with the "Standard" Shield Block uptime, then this would be the following averaged result over 15 seconds:
(9 x 0.5082D) + (6 x 0.2940D) / 15 = 0.42252D overall average over 15 seconds
Now it's already been accepted that stronger hits (high values of D) make Shield Block better, while weaker hits (low values of D) make Shield Barrier better. Where is the break-even point? Well, with 10 boss attacks in that time, and with one use of Shield Barrier absorbing 40,000 damage in the same span, we have
10x0.42252D = 10x0.5082D - 40,000
D = 46,685.3 damage, per hit
Let's put this value in perspective. On a Morchok 10H run, with the current 25% nerf, Morchok is hitting most DKs for about 40 to 45 thousand damage, INCLUDING the fact that he's using a Stomp debuff on the tanks that causes them to take +50% physical damage, about one-third of the time. Because warriors use shields, their armor is a little higher, and take a little less on a "hit". D, on the other hand, is HIGHER than this value. So, this means we are talking about a boss that is currently hitting harder than even a 10H boss while you have a significant damage taken debuff on you often.
With me so far? Good. Now let's crank this fight up to "Max" Shield Block uptime.
(3 x 0.5082D) + (12 x 0.2940D) / 15 = 0.33684D overall average over 15 seconds spamming Shield BLOCK
10x0.33684D = 10x0.5082D - 80,000
D = 46,685.3 identical. In other words, Rage income seems to have no bearing on the Shield Block vs. Shield Barrier debate SO FAR. We'll come back to that topic later.
Now so far, this looks bad for Shield Block. But it gets worse. Here are my real stats on the Beta: 41.64% dodge/parry/miss against a raid boss, block chance 24.62% against a raid boss, and critical block chance 61.70%. These changes are relatively minor, true. However, I was able to generate enough Vengeance to get a 66,000 Shield Barrier by fighting level 80 trash monsters, and in a raid situation, my Attack Power couldn't be any lower. Following the same template as last time, we see:
-- without using Shield Block, the average damage is now 0.51388D
-- while Shield Block is active, the average damage is now 0.30049D
-- using "Standard" Shield Block uptime, the damage over 15 seconds averages to 0.42852D. Compared to a single 66,000 Shield Barrier, this gives
10x0.42852D = 10x0.51388D - 66,000
D = 77,319.6 damage. I will remind you that this is based on my level 85 stats. This much damage will three-shot just about any tank in the game, and those that survive are barely standing. Heroic Morchok 25 is doing about 70,000 average damage per hit these days, including the aforementioned +50% damage Stomp debuff uptime. What I am saying, is at this time, pretty much the only bosses capable of doing this kind of pure damage per swing, are the 25-man Heroic DS bosses with the 25% nerf removed. This is not just "hard-hitting". This is the top of the top, the things that not even 1% of the entire playerbase does. I'm not a PvP expert, but I'm willing to bet that 77k melee hits don't happen very often in PvP, and if they do, then I'm glad I don't participate.
-- using "Max" Shield Block uptime, the damage over 15 seconds averages to 0.34317D
10x0.34317D = 10x0.51388D - 132,000
D = 77,324.1 damage, difference explained by rounding. Once again, Rage income seems to have basically zero effect on the Shield Block vs. Shield Barrier dispute SO FAR.
It is also worth noting that, in both cases, the value of D was just a little over the value of the Shield Barrier. Think about a time when a tank is in trouble, is taking a lot of damage and needs help FAST. In such cases, popping Shield BARRIER will prove a smarter move, as even if the next attack is a completely unblocked hit, a very large percent (over 85%) is absorbed, and the attack can still be blocked or critically blocked. Popping Shield BLOCK, on the other hand, would at best reduce the damage by 60% assuming the attack is critically blocked, which is not 100% of the time, and there is no backup afterwards. If the tank is low on health, there is no question.
Here are a list of things that make using Shield BARRIER and even smarter idea:
-- any bonuses to Attack Power you can get, especially Vengeance, raises the Shield Barrier value and, therefore, raises the value of D. You could expect to see a large number of warrior tanks using Battle Shout because of this. Worse, you can expect to see warrior tanks USING PVP/DPS GEAR ON PURPOSE and claiming it helps them survive, and sadly, they'll be right.
-- any bonuses to armor. Armor lowers all incoming physical damage, and reducing the damage of the blow makes Shield Barrier a better move. There aren't a lot of bonus armor effects or items, and most tanks don't seem to think highly of them, but technically, they make Shield Barrier better.
-- any unblockable physical attacks, such as Heroic Morchok's Stomp attack. Most tanks in most Morchok fights take more damage from Stomp than from melee attacks, based on the logs I was able to browse. Other common examples include Blackhorn's roar and the Madness Impales.
-- any form of magical attacks, which cannot be blocked at all, make Shield Barrier a MUCH smarter choice.
-- any "chance on hit" effect that applies on a blocked attack, but not on an absorbed attack. Nothing important leaps to mind in this category, however.
-- healing debuffs make Shield Barrier more attractive, because while Shield Block causes you to take less damage, Shield Barrier causes you to take none at all, buying your healer more time. The only exception is a healing debuff coupled with really high-damage attacks, also known as "instant death".
-- high-rage fights. Shield Barrier has no cooldown, nor does it have a lockout on its maximum uptime, which Shield Block has. Shield Barrier can also be used with under 60 rage, allowing you to top up nearly at will, which Shield Block can't do, either. Rage bursts caused by lucky procs and shouting can always be dumped into Shield Barrier, but can't always be dumped into Shield Block.
-- higher avoidance levels reduce the number of times an attack could be blocked at all, which not only reduces the value of Shield Block but of mastery as well. For instance, if you use the first set of stats mentioned, but replace the 40% total avoidance with 50%, then D jumps to 56,022, roughly a 20% increase in the amount of damage per hit needed for Shield Block to remain a good idea. This is roughly the amount of damage Heroic Morchok 10 was doing without the nerf in place.
Here are some things that make Shield BLOCK a smarter move:
-- bosses whose melee attacks have a "chance on hit" effect that fails on a block. I honestly can't think of a lot of those off the top of my head, maybe Blackhorn's Devastate or the bloods on Cho'gall.
-- fights with INFINITE Rage mean you can use both Shield Block AND Shield Barrier. However, at this time, Rage is not generated from being attacked, so barring odd mechanics like Vael, you won't see this, either.
-- high values of mastery make blocking better, because that's simply what mastery does. However, they'd have to be REALLY high. The honest stats I mentioned above involve me stacking mastery fairly heavily already. Eating a +90 mastery food in the Beta caused my Block chance to increase by 0.25% and my Critical Block chance to increase by 1.11%. Assuming both of these figures are linear (which, because Block has diminishing returns, they can't be), if I somehow got my hands on +500 extra mastery at level 85, then the value of D would drop from 77,319.6 to 75,862.1 which is a drop of about two percent. And this is the equivalent of ten whole epic gems ALL stacking mastery! By contrast, +500 strength would increase my maximum Shield Barrier by 2,500 (based on the scaling I was able to find), which in turn would change D from 77,319.6 to 80,253.1, a change just shy of four percent, i.e. almost double the effect in the other direction.
Conclusion: By the current mechanics, using currently existing raid models as a comparison guide, the only players who would ever want to use Shield Block are those who are fighting the very, very hardest of encounters. The vast majority of all warrior tanks, by current content, will never fight a boss capable of doing the kind of damage that is needed for Shield Block to outshine Shield Barrier. Therefore, almost all warrior tanks will spam Shield Barrier at all times and never use Shield Block, regardless of the context. Worse, the mechanics also not just allow, but encourage the same tanks to gear, gem, and enchant for Attack Power rather than defensive stats or mastery.
In other words, the current situation of Shield Barrier is teaching warrior tanks to be bad.