Here's the trick about health that has been heavily masked by the instance design in WotLK endgame:
In theory, there is a practical cap. The specific value of that is tricky, and currently the functional range of variability is so significant that it is hard without the ICC buff to *really* be on the sunny side of that range. The practical cap can be described as the amount of damage you can readily expect to take, in a given window, in which you will not receive (sufficient) healing to overcome.
That sounds vague right? The point is, you cannot express health in terms of *never* getting healed, because if you never get healed, you die, period. You cannot express health *really* in terms of an excess of required healing because then you will not die unless you cannot survive one hit.
WotLK has masked our ability to appreciate this value because the balanced range of damage is pretty usually 30-50% of our total health. The best geared tanks with pure Stamina/EH focus, in the 30% raid buff, can push that portion all the way down to 25-35% for the constant melee swing pulse. That still means that 3 hits will kill you which can vary that ideal health target with a huge window.
Here's the catch though, when we go into Cataclysm that proportion plummets. It allows us to find a much more precise range in which you can meet your target, and above which you can comfortably say you have "enough" health. How do you figure out that range? It is simple so long as we aren't too specific. =)
1.) Understand what is the largest "spike" of damage you can take (consider Gortokk the Impaler, when his melee lines up with applying a stack of the bleed, *and* with a bleed tick, big mass of damage from differing phases)
2.) Understand the window (usually determined by the rate of healing determined by cast times at the minimum for reflexive
healers, determined by mechanics otherwise as far as distracted/moving/incapacitated healers).
3.) Understand the effective value of health. EH, woot!
#1 we can figure out worst case scenarios for damage scale on a time basis since Crushing Blows are no more in raids, and we are not crittable. That means you just need to consider the elements of the fight with only minimal occasions of varying damage value (like stacks of Magic Buffet).
#2 is a little trickier; this is where there is a fudge factor and a bit of guess work. There are situations where nothing you do will save you because of the particulars of one attempt. Other times you didn't need a safety net because your group was fully on top of it. We cannot gear for infinity (cannot be saved), but we don't gear for when everything goes right. WotLK gives us situations where we have to consider the cast times of healers, and a lot of healers, sadly enough, are still reflexive (meaning they respond to your need for healing after you take damage) or they spam blindly which is just flipping a coin for effectiveness. For the record, the best healers can anticipate damage and defy that minimum window because they've already started their cast before you take damage. These sorts of healers are few and far between though, especially after Wrath. You can set reasonable windows though, or use your EH value to create a window for an encounter, a "time-to-live" sort of calculation (again: note that it is encounter specific).
#3 is an older concept that has had some more recent renaissance. The take-home message is to remember that the value of health is inflated through the mitigating abilities. Recognize that the value of your health scales with damage reduction, the more reduction % you have the farther each point of health takes you.
In Cataclysm two key elements change:
1.) Hit size proportional to health scale drops. We still take damage but health burns down instead of flipping on and off.
2.) Healers do not operate in max HPS mode at all times
The second portion is important here too. Healers will be using their efficient mode by default, and will be able to flip into high-power mode when they need to. They cannot sustain high-power mode at all times, so they will only employ it when they *need* to. This means that a big lump of damage will not be *SURPRISE-dead* it may drop you low and the next hit may still not kill you, but the healer(s) have the ability to dial up their healing on you for the counter.
All of this softens that threshold. Health does not lose its value, it just becomes easier to see the finite value. The *only* class that has a good exception for that is DKs, but that is something that will take some mathing. The only reason we are different is because of Life Shield, but seeing as how that is no longer based on our health, and we are generally expecting the actual value to scale with damage, we are lifting that reliance on health even for us.
Let's do a quick practical demonstration though. First an extreme.
Let's say you are taking 20k hits every 2 sec, and a 40k breath every 15 sec. For the sake of argument, lets say you don't have any tools like AMS to make that breath a non-spike.
For a low extreme, if you have 45k health, two swings (5.99 sec window for reflexive healers) will bring you very close to death, as will the breath. If you take a breath and a swing at the same time, you're dead. Definitely too little health.
For a better targeted minimum, if you have 65k health you can survive a breath and a swing at the same time, but you'll have at best a ~2 sec window to be saved or the next bit of damage will kill you. Melee swings will feel a little nicer than they do in ICC right now since it takes more than 3 to kill you.
For an extreme high, if you have 150k health you can take 3 swings (6-8 sec) with no avoidance, healing, or other tricks, take a breath hit full in the face, and then still be able to survive 2 more swings (4-6 sec) with a decent margin of health left and not die. Would anyone argue that you need that much health, or that it is really helping you that much except for that situation where you're trying to pull out a win when your healer(s) die?
Relative to the damage I listed above, ICC falls between the first to examples. If you aren't pinned to the top in many of the fights, the next breath will kill you. More health means you can reduce the chances of an unfortunate sequence of events, like a healer or unable to heal, combined with a lack of CDs at the right moment,, etc. That said, it is hard to find that turning point where it becomes too high because of the scale of damage. If you can take 2 hits and a breath without dying (say 85k) that gives a 4-6 sec window in which healers can make sure you don't die in the worst situations. Even for poor/slow healers, a heavy heal takes ~2 seconds to fire off and in ICC can push you back up to full.
Now consider a new scenario, like 85 stuff. Relative to that same damage, you can readily hit 105k health. That means you can survive a breath on top of 3 hits, and not die (6-8 sec window). As opposed to ICC, healers will not normally be filling you up with every cast, but they can do a serious chunk of that when they *need* to. The less they have to the safer you are, and the way to reduce their need to put heavy healing on you is by improving the tools you have that reduce the damage you take.
Now consider, do you add:
A.) 10k more health
B.) take 1% less damage
-or maybe as is more likely the choice-
C.) give yourself another 1% chance not to take each of those hits
Adding more health seems less valuable when this is a worst case contingency, no? Hardly without value, but the higher you go relative to the damage you take, the more it feels like an insurance policy you may not use.
The feel of content makes all the difference in the world, improving the stat value on health doesn't really improve its value relative to anything else, it just makes it easier to hit that point beyond which you don't feel like you need more health.
Personally, I feel like I'm beyond set with 70k working health in ICC, but plenty of people think that I would be better suited if I had heaps more because health is King right? =) I feel like I'm on the sunny side of that desired value because I am *never* the determining factor of a wipe (and frequently get yelled at for dying too slowly when we're trying to reset).
The thing is, I cannot give you a concrete value with solid math behind it, but Clysm offers the possibility that we may actually be able to create a more concrete range, possibly expressed in terms of "the most stringent tank fight would require a max health of 120k to survive a 5-8 sec window." The specific value to your group would just require a good sense of how good your healers are at their (now more interesting but more complicated) job.