Sorry for the long wall of text, but I wanted everyone to be on the same page.
Think about it this way.
Avoidance is good because it removes a lot of damage. Avoidance is bad because it is unpredictable. If you stack too much avoidance, you are likely to give your healers coronaries.
Mitigation (armor and straight damage reduction) is good because it's consistent. As you all point out, you can start to learn how much a blow will actually do to you. Mitigation is bad, from a player's perspective, because it can't save you. If you have 10 health and dodge, you might live. If you have 10 health and hope your armor will save you... well, it won't. You become the dreaded mana sponge because you are never avoiding damage completely.
Mitigation also has a risk from a design-perspective that when fights get too predictable they become too easy and unexciting. Imagine a tank with 75% damage reduction and no avoidance. You could calculate from the moment of the first attack whether you will survive the encounter. Heck, you might be able to not even heal the tank and know you'll survive depending on the specific abilities used by the boss.
Block as a mechanic is somewhere between avoidance and mitigation. Ideally it removes a fair amount of damage (vs. all damage) reasonably often (vs. rarely). If block is up 100% of the time it just becomes armor that you improve through a different stat. We have let block chances creep up frankly because the amount blocked is pretty trivial when bosses are hitting for 40% of your health pool every swing. If this still strikes you as too RNG, imagine abilities like Shield Block and Holy Shield that could guarantee 100% chance to block for a short period of time.
We don't think block is cutting it as a mechanic, but the direction we are likely to take it is probably more of a change than you are considering.
We also don't think it's necessary that every tank rely on avoidance, block and mitigation in equal amounts. They can't get too far apart or someone will come to dominate for certain encounters, but we don't think the tanks need to be completely homogenized to get what we want either.
If (to make up numbers) the DK and druid get hit for 20K every swing that hits, but the warrior and paladin get hit for 24K half the time and 16K half the time, then that seems like it would work. When the boss emoted that his big hit was coming, you could make sure you had your cooldown ready to guarantee a block.
Take my examples with a grain of salt. I think some of you are trying to plug them in to today's encounters without changing anything else.
I agree that nobody wants to be the tank that avoids, avoids, avoids and then gets hit for 4X normal damage. (Then again, part of the problem we had with DKs last patch was their avoidance was just too high.) In my example, the shield-using tanks get hit for 4K more damage 50% of the time. If you're calling that "spike damage" and saying it's unacceptable, then I'm afraid nothing can be done to salvage differences among the tank classes.
Yes, this is why I said not to assume everything else stays the same. We are in a world where almost any healer can heal a tank to full health in just a few GCDs. When healers can heal half your health pool, then bosses have to hit very hard. The test of survival for a tank is whether they can survive two hits or so without a heal landing. Health pools probably just need to be higher. Currently being the mana sponge tank isn't very scary because healers don't realistically run out of mana on most fights and the only real threat is whether you can get the tank back up to 100% before the next hit lands. Mitigating damage isn't seen as something that preserves healer mana. It is seen as something that might let you live through one more hit.
In a world where taking a little less on some hits and a little more on others doesn't translate into scary spike damage, then we think the block mechanic described above world work in some form, especially if you could force a block for those times when you did get a string of unblocked hits.
Crushing blows hit for 150% of normal damage. So a 20K hit normal hit would be an unpredictable 30K crushing blow. If crushing blows were 120% of normal, but you also had an equal chance for an 80% of normal hit, then it would even out in the end... provided that the spikes were not too big.
It may also be that the blockers would need to take even less damage when blocking because there is some merit to the argument that the tank with the lower variance on damage is easier (or at least more attractive) to heal even if the averages came out the same.
However, we are still just going to reject the notion that anything with an RNG component is unacceptable (provided you have the tools to occasionally get by when you get unlucky). I can totally understand as a player why it's in your best interest to minimize the RNG when tanking. You can also understand, I hope, why it is not in our best interest.
As I said above, once you can totally math out how much damage you're going to predictably take at the start of the fight, it will absolutely be an easier fight to tank... probably to the point of boredom. Imagine the 0% avoidance, 90% mitigation tank in a land in which bosses can never crit or crush. Is that going to be an effective tank? Yes. It it going to be fun to play? I doubt it.
First off, what's the actual state of affairs right now? Well, right now the example is more like this:
Druids & DKs take around 18k a hit, and sometimes (if a druid is lucky) 16k.
Paladins and warriors take around 24k a hit, and sometimes they'll take 22k.
Clearly that's not reasonable. And yes, this is with all the relevant factors of mitigation thrown in. The fact is, with things hitting as hard as they do block at a static component is not useful compared to armor. I showed this in the SD math post a while back; SD is great against small hits and weak against big ones.
So the situation is that warriors and paladins take both more incoming damage overall and more spiky damage. Some of this is offset relative to druids with slightly superior avoidance, but even that's more spiky. The big worry about block being too good is clearly wrong. Block isn't too good. In a world where blocking isn't required because crushing blows don't exist, armor is king.
The thing is - we already knew this from BC. We knew that on any fight where the boss couldn't crush and where there wasn't some inane 'must block' mechanic, a druid was going to be better due to armor. Well, druids have less armor than ever before but they still have more than any other tank, and most everything else is equal. Is it a surprise that this rules the roost? Same goes with DKs - it shouldn't be surprising that they're better off in pure mitigation on the boss fights in Ulduar.
And I'm not alone in saying this; it's been talked about on the forums, at TankSpot, in a really good overall article by Allison Robert at WoWInsider, etc.
So. Block is broken. GC recognizes it. It's pretty clear when there are a lot of comments by GC that changes are upcoming, even if they've not been finalized. We have seen this before with things like druids being able to use all weapons and get FAP no matter what, or with the trinket armor being a problem, or with savage defense. This is the first stage - sounding ideas off the community to let them know that something is coming down the pipe.
I suspect that we'll see an actual mechanic change here in the next couple of weeks. The question is, what will it be?
A lot of people on that thread are saying that it's unfair that the 'spiky' tank has less HP than the other tanks, and are upset that GC isn't commenting on this. I think that this is just because Blizzard hasn't solidified a design for all the tanks in this 'block is good' world. Why not? Because there are a LOT of things to consider. Let's go through some of them and see if we can figure something out.
- Block as a static mitigation fails against hard-hitting mobs. We knew that already.
- Block as a scaling mitigation fails against soft-hitting mobs. Savage Defense was a great tool for upcoming druid tanks in heroics precisely because it mitigates so much more damage against heroic-level attacks. Expect this to stay fairly similar to what it is now, because most tanks don't go to raids right off; they tank in normal and heroic content first.
- HP on the steady tanks should be lower than HP on the spiky tanks. This right now is a big deal and is very problematic. Druids are arguably the least spiky tank thanks to the higher armor and SD, and they also have the most HP of any tank. Clearly the less spiky tanks should have less HP, which would put paladins and warriors as the high HP tanks and druids as the low HP tank. That may be unacceptable flavor wise for druids.
- Magic mitigation on the high-HP tanks needs to be less than the low-HP tanks. Because if you can't block at all and armor doesn't help, everyone takes the same damage - which means raw HP is key. We saw this with druids in Sarth3D. I don't think they want this again.
- SD is in an odd state. I don't know what they'll do with SD. It has a high uptime but is similarly ineffective against hard hits as block. I expect two things to happen: SD to be dramatically improved and the HP of a druid to scale dramatically worse. The alternative is a reversion of SD and making druids and DKs the 'nonshield' tanks and paladins and warriors the 'shield' tanks. I'd actually like this, even though I like SD; it would mean that (similar to healers) tanks would have something of niches again. It would necessitate actually having some ability that scaled with DPS gear.
- Avoidance on all tanks should be roughly similar. This is already largely the case (within a few % depending on build) so I don't see it as an issue. And avoidance stacking is not nearly the problem it was in TBC.
- Blocking tanks must not be able to get 100% block uptime. This represents another drastic change to basically every single tank. Now why is this necessary? Because if you balance around the idea that half of the time, another class will take 10% more and half of the time they'll take 10% less, if you can skew that so that 100% of the time they'll take 10% less...that won't be balanced. Paladins mechanics will be completely changed (and likely to something near the warrior model with shield block). This means gear with block rating needs to be carefully looked at. It may mean that block% will have to have diminishing returns like avoidance does.
- Block value must be completely redone. This seems obvious. At the same time, you can't have this scale too amazingly well. You might not want it at all or want it to be gamed at all; being able to dramatically influence your scaling mitigation is a pretty powerful ability. You run the risk of block value being like a TBC druid's armor stat, where anything without it is essentially pointless. And then there's the scaling BV with strength.
- Threat stats will have to be looked at as well. Both paladins and warriors scale their threat with strength and block value, and have many mechanics built around these two values and how they interact. If they are changed dramatically, these things will have to be revised as well.