This is an excerpt of a conversation I had with another Tankadin on Altar of Storms.
We were getting ready to start a boss fight and I didn't think I could really explain my reasoning well in 25 seconds.
This conversation was talking about Gems specifically because it's one of the few places where tanks get to make an active choice of Mitigation of versus Avoidance. Specifically to this conversion we were discussing stacking Dodge gems or Stamina gems.
Tanks debated this in Vanilla WoW, they continue to debate it in tBC, and I'm sure they will debate it again in Wrath.
So allow me for a moment to outline why I feel that Stamina gem is the superior choice.
The 5 seconds that kill us are more important to us than the overall damage reduction during the 5 minutes we're in combat.
There are really only two ways to survive those 5 seconds. Either have enough health, armor and block value to survive the hits, or have enough Dodge, Parry and Miss so you don't take all those hits.
I argue the first approach (health, armor, and block value) is better because you never have to wonder what the RNG (random number generator, i.e. dice roll) is going to say.
Dodge, Parry, and Miss make the worse case scenario less likely. Stamina, Armor, and Block Value make the worst case scenario survivable.