Before you read this!!!! The primary purpose of posting this is only to stir debate about stat stacking as a tank. For example, I saw that both Prydain and Nemesis are stacking stam alot in their bear between trinkets and gems. While it appears that this is still a viable strategy, a primary focus of each thread was pointing out that mastery, for most classes, does a good job of smoothing out the damage that you're taking. All that means is that, while mastery and other avoidance stats may provide a supposedly "RNG" damage mitigation--and let's be honest, it is RNG--they do a good job of making the damage more consistent. Consistent damage is easier to predict and, therefore, easier to heal. I'll let you make the decision for yourself, but I have found that I consistently take less damage than both Prydain (on the cho'gall fight we did on monday) and Nemesis, here and there.
Death Knights: From EJ: http://elitistjerks.com/f72/t110102-blood_%7C_dk_endgame_tanking_%5B4_x%5D/#Stats_and_gems
More speficially,
"Stats and gems
"Parry and dodge have the same rating and diminish at the same pace. They're equal now.
Reforge for Mastery, unless you need to reforge for hit/expertise (threat).
"How to gem is not yet conclusive, however there is a strong argument developing that you should bias avoidance and mastery rather than stamina.
"Runeforge: possibly swordshattering rather than gargoyle.
"There is no consensus yet on how to measure effective health, and most importantly, at what point EH ceases to be relevant. Mastery as a stat does not increase your classic EH, but increases your survival, as does avoidance which will be a more interesting stat choice in Cat than it was in WotLK. There is a long discussion here including maths, tables and formula if you'd like to contribute."
Warriors from EJ: http://elitistjerks.com/f81/t110315-%5Bcataclysm%5D_protection_warrior/
Specifically:
"Survival Stat Priority:
"Stamina > Mastery > Parry > Dodge
"Stamina is still your best stat, but not to the extent of sacrificing everything else as was the case in WotLK. Conserving healer mana is a very real concern now. Mastery is your best stat for smoothing out incoming physical damage, and also increases rage generation through blocks. Parry provides the same avoidance level as Dodge now, but also procs the Hold the Line talent. Dodge will become more valuable as gear ratings climb higher to reduce diminishing returns losses from over-stacking the other stats."
Paladins: http://elitistjerks.com/f76/t110338-cataclysm_tanking_protection_field_manual_updated_feb_9_2011_4_0_6_a/#Stat_Overview
Specifically:
"Now for a word or two about reforging. Reforging will allow you to shift stats around on gear to (hopefully) optimize your build by moving 40% of a secondary stat to another (even not present stat) on the same item. What does this mean for tanking Paladins? Now that blocks are no longer a fixed amount of mitigation but instead reduced damage by a certain percentage it is in our interest to try to block as much as possible. As such we want to reach the 102.4% total avoidance threshold (also referred to as "block cap" or "complete combat table coverage") that we have aimed for in the past. This pushes regular hits off the table so that any physical hit has to come through a block. Given that tanking Paladins should be reforging for Mastery as much as possible. Note though that is it unlikely you will be able to reach the 102.4% threshold early in Cataclysm. Until post 359 item level gear and epic gems are available ratings will be too low. Regardless, you want to get as close are you possibly can to the 102.4% threshold."
"The reforging (and gearing) strategy are aimed toward maximizing mastery to maximize your block chance. This is important for Paladins because being block capped smooths out the rate of incoming damage, reduces damage spikes, and makes it easier for your healers."
Finally, Druids (this is actually one of the sites I skimed that actually supports the above but only because thenature of your mastery is sketchy): http://elitistjerks.com/f73/t114069-%5Bferal-bear%5D_cataclysm_release/#Gems
Specifically:
"Your only real meta gem choice for tanking is [Austere Shadowspirit Diamond]. In specific cases you may find [Effulgent Shadowspirit Diamond] or [Powerful Shadowspirit Diamond] useful, but generally speaking Austere will be just fine.
For the rest you'll want stamina or hit for blue gems, crit, dodge, or mastery for yellows, and agility for reds. If you're ever in doubt you can default to [solid Ocean Sapphire], but be sure to keep your meta gem activated. Use Rawr or the spreadsheet to help you figure out what you need."