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.
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.