Some people might not like Allison Robert on wowinsider, but I think she is incredibly intelligent and often gives some of the most sound advice to those that might want to fulfill a role in a group.  Here is a c/p of a recent post by her (warning: HUGE post, but fantastic read):

Shifting Perspectives: Tanks, "Wrath," and crushing blows

by Allison Robert { 15 hours ago }

Every Tuesday, Shifting Perspectives explores issues affecting Druids and those who group with them. This week, we examine the roots of the uproar over the proposed Heart of the Wild nerf, and also ask ourselves if it wouldn't just be easier to reroll a Death Knight and have done with it.

"Why would you title the column this way?" you ask, as you reach for your "Please fire _______ from WoW Insider" form letter. "Crushing blows are out of the game, dipwad."

Well, yes. The crushing blow is technically out of the game, but another and worse mechanic has taken its place.

In this article I'm going to try to explain the source of "shield tank" frustration over health pools -- and why they are correct to see it as a problem -- and the Druid tank's unhappiness over the nerfing of Heart of the Wild -- and why Druids are also correct to see it as a problem.

Why the crushing blow was important

One of the biggest differences between pre-Wrath and Wrath tanking is the absence of the crushing blow. If you're unfamliar with the term, then as a very simple explanation: any given raid boss had a 15% chance per melee hit to perform a 150% damage attack, which was also known as the crushing blow. It was typically a big damage spike and could lead to a wipe on progression content, with healers struggling to compensate in the small window of time before the boss' next attack landed. Burst damage is very unwelcome as it's often the greatest contributing factor to tank death. This is why reaching crit immunity is still so important to all tanks, and why the ability to avoid or absorb crushing blows was a fundamental part of pre-Wrath tanking mechanics.

The only means of avoiding a crush was pushing your dodge/parry/block over 102.4%, which would nullify the boss' higher weapon skill and render the tank immune to crushing blows. And this is how tanks dealt with that individually:

Warriors would use Shield Block to push their dodge/parry/block over 102.4%.

Paladins would use Holy Shield to push their dodge/parry/block over 102.4%.

Bears would eat it. Prayer was often involved as well.


Bears have no form of avoidance beyond dodge. Because crushing blows were the last attack to be pushed off a boss' hit table, a bear would need more than 85% dodge even to avoid some of them. While a gear set with >85% dodge was technically possible in late Burning Crusade content (and something similar was used by Rogues to tank bosses with predominately physical damage, which is why you saw all of those Rogue-tanking videos popping up in late BC), the amount of agility you needed to pull it off left you with a tiny health pool and almost no rage generation. You wouldn't get hit, but you also wouldn't be able to hold aggro, so it was the sort of catch-22 that made the situation an amusing one to consider, but then you'd laugh and go back to your regular tanking set.

Without the ability to avoid crushing blows, a Druid's armor and health had to guarantee that: a). the blow would hit for less, and: b). they would have the health pool to absorb multiple, even back-to-back crushing blows, because there was nothing the Druid or raid could do to change the boss' 15% chance to crush.

The Druid's overall damage taken was thus heavily reliant on RNG. You might go minutes or more on a raid boss without once being crushed -- and then there was the time on Teron Gorefiend where I was crushed on six successive attacks and we wiped. The only thing that could prevent that was better armor and more health, so Druids pushed their armor to reduce the damage, pushed their agility to avoid as many normal blows as possible, and then pushed their stamina to outlive the attacks.


The approach of Wrath and the new tanking paradigm

One of Blizzard's stated aims with the introduction of the Death Knight and the overhaul to existing tank specs was to address the chronic tank shortage on most realms. They did this by reworking the Protection Warrior tree extensively, introducing the ability to frontload threat and CC in the form of Shockwave, generate better reflective damage, and improving Thunder Clap to function as a relatively good form of burst AoE threat. Protection Paladins were altered to have better single-target threat and more controllable aggro generation, which somewhat addressed their relatively weak off-tanking capabilities. Bears overall were changed least in their basic threat mechanics, but were given occasional burst AoE threat in the form of Berserk, the removal of a target limit on Swipe, and cooldowns in the form of Barkskin in forms and Survival Instincts (a clone of Last Stand).

The net effect was to improve tank damage, soloing capacity, and threat scaling in an attempt to make tanks a little more user-friendly and fun to play. I would argue that in most respects Blizzard has accomplished that goal.

The Death Knight

Death Knights were also meant to allay the tanking shortage by providing a new, cool-looking, and predictably popular class that began life at level 55 and utilized several unique mechanics.

However, the introduction of a 2H-weapon tank who lacked the ability to block was a problem for the concept of the crushing blow. In the absence of block, Death Knights would be unable to become uncrushable without increasing their total avoidance from parry and/or dodge beyond 85%. Increasing their armor or health to the level of a Druid's would also have made them the single most powerful tank in the game against any form of damage, which was problematic for Blizzard's efforts to standardize tank quality. Developers were probably already concerned about the potential effects of Death Knight tanking talents on PvP balance. As we later saw, the Death Knight's impressive mitigation and avoidance cooldowns, coupled with frightening damage and self-healing, has made them an unwelcome and often resented opponent in battlegrounds and arena.

If you cannot change the class to suit the game mechanic, then the solution is to change the game mechanic to suit the class. Crushing blows can now be performed only by NPC's 4 levels or higher than a player character. As the highest-level NPC you will encounter in a 10- or 25-man raid is level 83, crushing blows functionally disappeared from the game.

Sort of.


The reappearance of the "crushing blow"

A crushing blow by any other name is still a "crushing blow" -- it's just not a crushing blow as we're accustomed to understanding it. When I say that the crushing blow has reappeared in the game, what I mean is that -- on encounters that matter, we continue to see enormous burst damage on tanks. The difference in new Wrath raid content (naturally we must exclude Naxxramas) is that the damage spike is typically magic in nature (e.g. Sartharion's Flame Breath, Malygos' Arcane Breath) which none of the three classic/BC-era tanks mitigates particularly well. Magic damage cannot be pushed off the boss' hit table and cannot be dodged, parried, blocked, or (for most raid bosses) interrupted or Spell Reflected.

Because the majority of your gear, talents, and abilities are completely useless versus this damage, the two things that matter most are your health (the more, the better) and your cooldowns (should you have them).

So what does each tank do about it?

Warriors eat it...which they are not designed to do.

Paladins eat it...which they are not designed to do.

Bears eat it, which is what we were designed to do when burst damage was nearly all physical in nature.

Death Knights use cooldowns to mitigate most of the damage, which is precisely what they are designed to do.


Why is this a problem?

Most of the time, it's not. In Wrath, the current raid content is sufficiently easy that any guild that works at it can usually clear everything given time. Sartharion's Flame Breath without drakes up is not particularly dangerous, for example. Blizzard is entirely correct in saying that you can clear all of the game's current content with any tanking class at the helm.

However, the current measure of a raiding guild is the degree to which they've made the content artificially harder by doing achievements. Your guild is no longer evaluated by the content you've cleared. Guilds and players don't advertise themselves by saying "MH/BT guild," or "3/6 Sunwell" anymore if they're after quality players. Hit the PTR around raid times for Ulduar bosses, and this is what you'll see in general chat:

"Twilight Vanq DPS LFG."

"Twilight Vanq/Nightfall Tank LFG."

"Immortal/Undying healer LFG."

This is how a smart player or guild will advertise themseves, both on the PTR and on the recruitment forums. If you want to compete with the best, it is no longer sufficient to advertise what content you're doing, because the greater accessibility of raid content renders it fairly irrelevant as an outward indication of player quality.


So what matters?

Achievements matter.

And the most important achievements overwhelmingly favor a Death Knight, or secondarily a Druid, tank.

The former has the cooldowns to manage the new form of "crushing blow." The latter has the HP to survive it.

A cynical person might say that early raid achievements (particularly Sarth 3D) were designed to make the prospect of a Death Knight tank more palatable to the entrenched tanking corps of serious raiding guilds. A very cynical person might say that early raid achievements were designed to force a Death Knight tank down the throats of raiding guilds that wouldn't otherwise want them.

Warriors and Paladins get shut out because their classes were designed to handle the kind of burst melee damage that is no longer in the game. Indeed, the whole notion of stacking dodge, parry, and block is intrinsically tied to the now-vanished melee crushing blow. The only reason that Druids are able to compete with Death Knights is, while our magic mitigation is poor, stamina doesn't discriminate against the type of damage you're receiving. We do what we've always done -- soak the damage and pray our healers have had their caffeine for the night.


The Druid problem

This is why many Druids find it upsetting to read gloating comments concerning the upcoming Heart of the Wild change, and why the nerf is a frightening prospect. I would argue that the increased -- and increasing -- difficulty we have stacking armor is the logical result of a world without crushing blows, and that Blizzard was right to scale us back. With so much armor, Druids faced the real prospect of being insanely overpowered against melee damage that's piddling in comparison to what we once survived. It's depressing to be nerfed, and still more depressing that it seems to be happening little by little and patch after patch, but it is also the correct approach.

You could make a fairly compelling argument that item points spent on armor for Druid tanks are almost wasted outside of appropriate scaling, especially when those points could go to +agility, or +dodge, +stamina, or (with the upcoming Savage Defense) +crit. We keep arming and arming ourselves against a huge melee blow that's just never going to come, and those item points could have been put to better use elsewhere, especially given the Druid's continuing problem with snap and AoE threat.

Do not misunderstand me. We still need armor, and a lot of armor at that, in the absence of parry and block. We just don't need it to the point of surviving that 15% chance per melee hit anymore.


Druids still have too much health!

Druid tanks are not the problem. The real problems are that:

a). The most important content in the game is ideally tanked by the only class (the Death Knight) designed to handle burst magic damage.

b). Two older tank classes (the Warrior and Paladin) are not particularly well suited to how guilds "progress" in achievement-oriented content.

c). One older tank class (the Druid), while not optimal for this content, is capable of surviving it due to an accident of design. Our health is being nerfed in 3.1, the scaling of said health is being nerfed, on top of yet another armor nerf whose compensatory mechanic (Savage Defense) does nothing to address the Druid's already poor magic mitigation.

That a voidwalker can substitute for a Druid on Sartharion 3D (and arguably does a better job) is no great recommendation for the mechanics of Druid tanking.


Do we give Warriors and Paladins more health?

It wouldn't hurt, but Warriors and Paladins are also designed as avoidance tanks. Avoidance is vastly superior to health in TTL (Time To Live) calculations for nearly every situation, barring that of the burst magic damage described above. Over the course of most encounters, Warriors and Paladins take less damage than Druids and Death Knights (although the former takes less damage per individual hit, and the latter is more likely to avoid said hit), and if you picked any random encounter out of a hat, a Warrior remains the best overall choice for it with respect to Time To Live. This is one of the reasons why Warriors are still the default choice for the main tank position in competitive raiding guilds.

To significantly increase Warrior and Paladin health pools without altering anything else, concurrent to nerfing Druid health, would come within shouting distance of rendering the Druid tank obsolete. Druids have traditionally stacked health in order to compensate for not having the avoidance of shield tanks and approximate their Time To Live count. That this enabled them to live through main-tanking a Sarth 3D is, as I've noted, entirely an accident. The encounter is clearly prejudiced toward a Death Knight tank.


So change Warriors and Paladins. More health, less avoidance.

That is precisely the set of circumstances that have made Druids the least favorite healing choice, and it's a particularly risky move with mana regen mechanics being altered in 3.1. Druids are already in the unenviable position of being the tank who guzzles the most healer mana. Do we really want to spread this considerabe weakness to more tanking classes in an age where healers will have to watch their mana consumption and Lifebloom stacks are probably out of the picture?

Do we bring crushing blows back and nix burst magic damage?

No, at least to the former. Unless you completely alter the mechanics of Death Knight tanking (which I suppose is a possibility, but probably an unwelcome one to developers), there is no way for them to become uncrushable. They would need a significantly higher health pool, and/or significantly higher armor, to survive crushing blows without being destroyed if a physical mitigation cooldown isn't up. They are the tank with the least physical mitigation and smallest health pool on average, and that would horribly magnify this weakness. But Death Knights are already a nightmare to deal with in PvP without giving them additional health, armor, or cooldowns. Therefore it seems fair to conclude that the return of the crushing blow would be a nigh-insurmountable obstacle for Death Knight tanks, and that the most obvious solutions would upset PvP balance still further.

Reducing magic burst helps Warriors, Paladins, and Druids. Increasing melee burst hurts Death Knights. Reducing burst magic damage seems a little more reasonable insofar as 3 out of 4 tanking classes have serious problems with it, but also makes encounters less challenging and fun from the perspective of both tanking and healing. A compromise of sorts might be reached with more, smaller magic hits being dealt over the course of a fight, or giving the older tanking classes additional activated cooldowns to survive Wrath's version of the "crushing blow." Again, from a PvP perspective this is an issue, but the Paladin in particular is hobbled with respect to an incoming, unavoidable magic hit.


Can we rework existing mitigation/avoidance mechanics to be more useful for Wrath raids?

Assuming that burst magic damage will remain the rule rather than the exception in 3.1 and beyond, allowing dodge/parry/block to slowly increase magic mitigation strikes me as potentially useful. I do not suggest that it necessarily scales to the point of outshining the Death Knight's particular strength, but it would be a valuable contribution to the survivability of the 3 "classic tanks" in Wrath raid content without measurably impacting PvP balance, as +defense, +dodge, +parry, and +block are unattractive stats for battlegrounds and arena and are unavailable on PvP gear anyway. Additionally, if this also increased spell resistance, it would help the Warrior/Paladin/Death Knight problem of remaining defense-capped in resist gear. The scaling would need to be tuned carefully, as each "classic tank" values these stats somewhat differently (Druids in particular are not well served by scaling with nonexistent +parry and +block, and little to no +defense) but it might be a usable option outside of fundamentally altering game mechanics elsewhere or allowing the "classic tanks" to struggle in encounters with heavy magic damage.

Of course, if Ulduar encounters and achievements are not built around the kind of burst magic damage that characterize Sarth 3D and Malygos, then this is probably unnecessary.


In conclusion:

From my perspective as a Druid tank, the problems of Warriors and Paladins on important Wrath raid content arise from how dodge/parry/block are still somewhat oriented toward the obsolete crushing blow. However, in the absence of the crushing blow, magic burst seems to be the most obvious choice to make tanking and healing require skill in Wrath raids, thus unhappily prejudicing encounters toward the only tank capable of mitigating such damage. That Druids are capable of surviving an encounter that is most ideally tanked by a Death Knight is due only to the usefulness of stamina in the very narrow context of constant burst magic damage.

I do not think it is a good idea to nerf Death Knight tanks for PvE content, but I also don't think it's a good idea to allow the 3 classic tanks to perform so poorly against this damage, or to nerf the Druid's last defense against all forms of burst. How all tanks mitigate and avoid damage needs to change to reflect how encounters have moved past one unwelcome form of burst damage to another.