So, I did a little number crunching about 2.3...

Avoidance becomes more useful than stam when you don't need any more stam. It's open-ended because it has to be, spike damage varies from boss to boss. If a boss can put out 20k damage in under, say, 3 seconds, then you need over 20k health, and if you can get past that point with an acceptable margin then you should start looking at adding more avoidance. If another boss can only do 12k damage in that time period, then you don't need as much HP and can work more avoidance in.
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In one sense what's been said here is the reason that healing in EQ was an amazingly different affair, in that game it was expected that tanking a raid boss, you'd be hit for 75% or more of your health in a second or two. Thus the complete heal chain was born with 3 clerics casting complete heal every 3-3.5 seconds so that they would land 10 seconds later. And in addition to the chain, a bunch more shaman and druids would be dropping incremental heals every chance they got, this basically meant that if a tank had the hp to survive for 1.5-2 seconds worth of max damage, the only ways the encounter would fail were healers going oom or healers making a mistake.

In particular, this was how the wow world worked in 72 man raids, or more people before there was a concept of a limited raid which didn't even get released until expansion #3.

WoW by contrast uses a smaller number of people and because the pace of the game is so different, healers are trained to react to damage. Also, wow bosses don't put out the same kind of damage as the eq bosses did back then, and healers are proportionally more individually effective.

What all this creates is laziness. In EQ if a healer wasn't on their game, the raid wiped. In WoW, if a healer screws up, there's often enough time to use an "oh shit" button, in eq, there weren't many.

When my group from eq came to wow we discovered a world of "ez mode", you didn't need a group to take an even con mob, individual mobs didn't take minutes to beat, death didn't cause you to lose levels, instances and quests meant ready and easy access to gear upgrades, using gear from 10 levels ago meant you were a noob, in eq, 15 levels ago was common.

This semi rant got off topic, but what I wanted to say was this, effective health and avoidance are great, earlier posters had it absolutely right when they said that different fights require different things, but at the end of the day after you've put on your gear and started the fight you are 100% at the mercy of your healers, and they are the ones who have to learn to anticipate damage, feel the pattern, and work proactively, and allowing them to slack and not be as on the ball as you is letting yourself and the 20 other people on the raid get screwed. EQ proved how coordinated people had to be, and WoW let them relax.
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There are 2 things a tank needs to gear for;

1) Incoming dps

If the healers cannot out put enough hps to offset the amount of incoming dps, the fight is over before it starts. An example of this is Moroes, he doesn’t hit hard, but he deals a lot of damage over a short amount of time, and he garrotes.

2) Burst damage

If the tank cannot survive a short period of time without heals, it may still be possible based on luck to complete the encounter without running into a worst case scenario. If you have that kind of luck, im taking you to Vegas and you can play the $100 slots for me .

Once you can equalize the incoming DPS with the incoming HPS then you can start working on surviving burst damage, but without both you might as well save your repair money.

There are many ways to reduce the amount of incoming DPS, and all ways have there advantages and disadvantages. These include armor, block value, and each of the avoidances. Causes of burst damage on the other hand are a result of not avoiding or mitigating incoming damage. This is usually caused by, but is not limited to, knockdowns, which almost eliminate avoidance and magical damage which goes around armor and avoidances. The only way to survive these kinds of hits is to have enough health and incoming healing that the effect ends before you die. (Resistances fall more into reducing incoming dps then taking burst damage, and relying on it, would be like relying on redoubt to keep you uncrushable.)

Both schools of thought are important, and both are required for successful tanking. Knowing when to have what though is more important then either by itself.

With that said, trying to balance your gear is probably the worst thing you can do. A tank that focuses on avoidance will do great on fights with sustained dps, and a stamina tank will do well against high spike encounters. A tank who tries to go for both will be screwed from both sides, unless he has gear from beyond that encounters difficulty.

(This is pretty close to the sentiment I’ve seen in this thread, it just hasn’t been said this way.)
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I was once a big fan of avoidance, and I have a long article written with some math that proves that avoidance is better. After about 5 hours of hard work on those queue systems and markov chains, the conclusion was so obvious: math will always tells us avoidance is better if you define "better" as "damage reduction". As gear scales, avoidance gives increasing results and armor gives diminishing (the life expectancy result didn't matter in my approach), and eventually you will reach a point of 100% avoidance, where you get immune to physical damage. Mathematically modeled, a 100% avoidance tank is better than a 100 million HP tank simply because the HP is finite and needs mana to heal, and mana is finite too. (not counting spells of course)

Then I realized, omg how obvious, World of Warcraft is not World of Mathcraft. I cannot model WoW perfectly without reprogramming the game entirely, and playing it with real people, because real people are not mathematical constructs, they fail, they are not deterministic, etc, etc, etc

So, I started to think differently: down to earth, my gear is constrained, and it already comes stacked with armor, stamina, defense, dodge. My choices are not open, Blizzard did 90% of them. The few options I have are on gems/enchant, either stamina or avoidance.

I chose to stack stamina because Kungen (back to the original topic ) did so, and because people I respect like Ciderhelm and Satrina promoted it so much, and I became an HP/Armor powerhouse, and it works. Gemming avoidance works as well I'm sure, and a real boss battle is very far from being determined by the tank gem choices, and much more by group tactics, raid members not making individual mistakes, being prepared, not taking a disconnect, and so on.

As much as the understanding the game mechanics might entertains us who love it, and I REALLY love to lurk in forums during work hours while my boss is away, in the end it doesn't make much of a difference in terms of game play results, and that is why its common to see many top tanks in top guilds who can't answer basic game mechanics questions.