Quote Originally Posted by Whowantstoknow View Post

Proper Gemming

Blue - Stam
Red - Agi/Stam (only enough to activate Meta - try to use in a socket bonus that is stam)
Yellow - Def/Stam (def/stam will always be better than pure stam if the socket bonus is 6 stam or better)
Meta - Stam/armor is your best EH meta so is the one most raiders will use but def/block value is superb for heroics and trash or where threat is a problem (block value increase ShoR damage)

There is no "defense cap" - defense will provide benefits beyond 540 defense (535 for heroics) it should be considered a minimum rather than a cap.

You should never gem parry
You can consider using Str/Stam for a red slot but agi is recognised to be slightly better all round
According to: http://elitistjerks.com/f76/t37172-p...8/#post1336981 which is a subsection of: http://elitistjerks.com/f76/t37172-p...wow_3_0_wotlk/

Red
(also orange, purple)

Good
Dodge rating (avoidance)
Parry rating (avoidance)
Agility (avoidance, tiny bit of mitigation, some crit)
Strength (threat, some mitigation)
Expertise rating(some threat, some avoidance)

Bad
Attack Power (always worse than strength)
Spell Power (even worse than attack power)
Armor Penetration Rating (only affects white damage threat and damage, which is tiny)

Red gems can have some useful stats for tanks, but there are no crucial red stats as there are for yellow (defense, hit) or blue (stamina). Strength can be handy for threat generation, but it's secondary in importance to hit, and the block value provided by strength is a fairly inefficient way to boost mitigation. For most tanks (as of the early stages of WotLK anyway), red gems are practically never used, and orange/purple gems are only used to activate socket bonuses.
The numbers for avoidance are nearly identical for AGI / Parry / Dodge, I think gemming for Parry is fine (I'm on the fence for dodge) I think if pure agi was the way to go you'd see A LOT more paladins stacking AGI over Stam. just like you see so many bears going nuts for AGI. Maybe I'm just too deep in the math to make sense of it.