There is some very interesting theroycrafting being done on Maintankadin right now about the definition of Effective Health (EH). Effective Health has been understood as the amount of damage it would to kill you, and has generally been understood to be a factor of your Armor, and Stamina.
The problem with the traditional definition of EH is that it doesn't really take into account any fight where the damage taken is from both magic and physical, which is essentially every fight in the game. It also doesn't model well physical damage that isn't reduced by armor like Gormok's Impale.
Paladin Theorycrafter and Math Genius Theck of Blackrock started looking at the definition of EH when he got involved in a discussion of armor trinkets versus stamina trinkets.
You can see the complete mathematical analysis in this thread.
Y is the percentage of magical damage, A is armor, H is health, M is mitigation, and N is a constant derived from the link provided.
Let's check some numbers. For a tank with H=50k, A=30k, M=0.6433. For paladins, N=0.1686. That gives Z=0.5709. Let's see what happens as Y varies from 0 to 1.
So for a purely physical fight, armor seems like a pretty good deal. But for even a fight with 20% magical damage, Armor becomes devalued by 20%. This will generally be enough to make a Stamina trinket provide more EH than an armor trinket.
For an example, let's look at the Glyph of Indomitability, since that's what started this thread. It gives 1792 armor, which is equivalent to 153 stamina. However, for a fight with Y=20%, we get only (1-Y)=80% of that, or 123 stamina. For a fight with 30% magical damage, it's only worth 107 stamina, and so forth.
This theorycraft isn't saying that Armor is useless. Armor is just less effective than Stamina as a means of getting EH in fights with a magical damage component. How much less effective will depend on the ratio of magical to physical damage. A fight like Lady Deathwhisper will have a much higher percentage of magical damage than a fight like Death Bringer Saurfang.
EH is an important concept for Tanks to understand. Warriors have been pointing to Paladins and Druids having an advantage in EH as a potential issue with the Warrior class, and have request the Developers investigate the issue. The Developers reply was that they didn't find enough of a difference to warrant either a Warrior buff, or a Paladin/Druid nerf. Some in the Warrior community were less than completely pleased with that.