It's not the amount of Stamina compared to secondary stats that matters. It's the amount of Health you gain compared to the avoidance you gain from the secondary stats. The stats behave in completely different ways, and Stamina simply increases your health more than Dodge and Parry increase your avoidance (percentually). That 100k health increase was based on my personal in-game experience when I switched my non-Stamina gems and two trinkets to almost pure Stamina at ilvl 470 or so (counting the bonus from one profession, and the other profession will increase it even further), so I know it to be a legit example.

An analogue to what you're saying could be something like "pizza can't be better than potatoes because you can get 33% more potatoes at the same price". It's not only about the quantity of the stat, but also its quality.

The real issue is finding out whether or not 20% more health is actually better than 8% more avoidance (note that I'm talking about percentages of increase, not actual percent points of avoidance). I'm pretty sure it is, but I don't have any hard evidence yet.

What do you mean "cannot be compared like that"? The only comparison I've made is how much Health and how much avoidance you can get by modifying your gear. I haven't made any comparison between their efficiency other than stating points of why Stamina might not be as bad as I've thought it is (because it isn't just survivability like you said, it also does all the other things I mentioned two posts back).

I don't think you understood what I was saying. You are comparing full stamina gearing to full avoidance gearing, but you should be looking at individual gear or gem trade-offs instead.
To make an extreme example, let's assume that for encounter "X", gearing for stamina is generally better than avoidance. Would you then drop 10000 dodge rating for 1 stamina? Probably not.
The point is that e.g. gemming for secondary stats might give you plenty avoidance already, leaving room to get stamina over secondary stats in the trinket slot. This would be much better than gemming for stamina and using trinkets with secondary stats because of the item budget.

One way to look at stat weighting is to look at your groups progression boss. In DS, my group found both Regular and Heroic Ultraxion to be that boss. Prot Warriors were able to have more hit, crit and strength for that fight as their block amount was incredibly high due to a tanking buff. I was able to contribute to keeping us out of the last minute of the fight, when raid damage peaks. We can have a better answer as to what we're supposed to be doing when we know what boss we are fighting.

It is a very old discussion you are reintroducing, which means most people who try to answer will not intuitively use a from-scratch explanation but instead give some comments that reflect the assembled wisdom of people who have discussed this many times instead of trying to make sure they explain things to the level you need.

I'm not particularly well placed to go to the bones of the matter and build it all up either, but I can at least help direct you. What you are talking about IS something you resolve with theorycrafting and a lot of it. Not theorycrafting in the sense of guesswork, but rather careful mathematical modelling and discussion about game/class/opponent mechanics.

The term for what you are talking about is "Effective health". It starts with health and is then modified by your mitigation/avoidance factors. Instead of talking about how much health vs how much avoidance (which is an apples and oranges comparison) effective health talks about how much damage an opponent will have to do to kill you.

For instance you can start with 500k health. Then you have 50% dmg reduction from armor - which gives you 1000k effective health. Then maybe you have a combined dodge/miss/parry chance of 1/3 (33.33%), which means the dmg that does hit is only 2/3 of what the opponent would do if he had no misses/dodges/parries - this means 1000k is 2/3 of your effective health - that your effective health is 1500k. Into effective health you can introduce factors for block/crit block as well - but also factors for healing and there is a mathematical truth and accurate way to model it.

So yeah - search for and read about effective health is my suggestion.

As others have said - health and stamina is something you never ignore as a tank - but it is also something you often assess and then decide is not your priority in every slot. This is because when it comes to health and stamina there is a matter of sufficiency. If you have so much stamina that you never go below 50% of max health while healers have mana in any fight that your guild is currently working on - but you still fail to kill bosses because your healers run out of mana healing you - then you have too much stamina.

This is meaningful because any reduction you take in stamina you shift over to mitigation or avoidance stats. So instead of taking 100k dmg per second after all mitigation/avoidance factors maybe you take 90k or 80k or 70k - so now suddenly the healers can keep up without running out of mana.

And it is meaningful to talk about stamina and health as one thing. Health is directly based on stamina. You'll always have the same buffs so comparing health and comparing stamina is the same thing. There are only really two exceptions - if you compare races tauren and pandaren can slightly change the valuation of health and stamina away from the norm - and if you don't want to end up in an apples/oranges situation but you want things comparable so you can assess them against each other stamina is what you use - because you can decide to use a stamina or a parry gem.

So that's why we talk about stamina - stamina is what health is called when we want to make a meaningful comparison between different ways of improving our effective health.

A summary for the world to see:
1) There is such a thing as enough stamina that you do not want to keep stacking it
2) This level can typically be reached by deciding to use zero, one or two stamina trinkets
3) On trinkets you get 1500 stamina or 1000 parry/dodge/mastery (1.5 times as much stamina as parry etc)
4) On ring enchants you get 160 stamina or 160 strength (which is approx 151 parry rating) - (1.06 times as much stamina as parry) - similar comparisons can be made for enchants in other slots.
5) In gem slots you get 240 stamina or 320 parry/dodge/mastery (0.75 times as much stamina as parry etc)

IF you reach the sufficiency point for stamina and therefore health with less than two stamina trinkets you should generally get your stamina on zero, one or two stamina trinkets because on the trinket you get a lot more bang for your buck. For enchants and particularly gems you should get mitigation stats.

I would say that for normal mode the choice for health sufficiency is between ZERO and ONE stamina trinket. It's possible for heroic the choice for stamina trinkets are between one and two. I doubt the sufficiency point for stamina even in heroics (especially 10 man heroics) is so high that enchants and gems have to be used to increase stamina in less efficient ways.

Also remember if using them for stamina - the brewfest stamina trinkets are really crap. A much better stamina trinket has both stamina and some kind of dodge/parry/mastery proc or use ability. Even if lower ilvl.

Alright, it seems that I wasn't clear enough with my posts. I took the principle of effective health for granted, and was merely trying to start a discussion on the "other things" that stamina provides. It's really the only thing that can increase our EHP (reliably) aside from armor, which we don't have any means to modify, so I didn't see any reason to specifically mention EHP. My thoughts are that Stamina and Armor make our EHP, and avoidance is just a way to help healers save their mana (they give "average EHP" but not actual EHP since they work based on random chance). Either way, Berthold's post in the new MoP thread confirms that I'm not completely out of my mind (where he mentions relative damage taken), and we should continue the discussion there if needed.

With that in mind, my previous posts might make a bit more sense.

Avoidance is best measured by the decrease in likelihood that consecutive attacks will happen. You might want to math up 2/3/4 swings in a row or something to get a good idea on the benefit you're getting from avoidance, but in past expansions the DR on it was really harsh to the point of discouraging any and all avoidance stacking.

"For fights where we're mainly using Sbar (Shield Barrier) we should gear; Stamina>Hit=Expertise" - Tengenstein - Tankspot. Warrior Harsh Words and Steel: A Protection Warrior Guide

It seems the answer is a 2nd set of gear. One with mastery/parry/dodge and another one with hit and expertise.