I forgot where I read this information, but please do correct me if I'm wrong or toss me some treads to look at for references.
The question I have is, I've noticed there are some rather nice items that'll give you more armor rating such as rings and trinkets. Now would it be better to use rings and trinkets to boost you armor and stam, at the expense of losing some mitigation, to raise your over all physical damage reduction?
(keep in mind that most of your tanking gear will already have mitigation on it)
"Avoidance" reduces damage by completely evading or negating attacks - parry, dodge, miss. "Mitigation" reduces damage by reducing the actual amount of damage that you take on a hit - armor, block. Stamina just lets you soak more damage.
Armor is good, but it has diminishing returns and a cap, beyond which no additional armor will give any benefit whatsoever. You're not going to hit the cap with gear currently in the game, if I recall correctly. On the other hand, because of diminishing returns, additional armor might not be the best investment of your item's budget.
The real question is, what else could you be using? There are relatively few trinkets, rings, and necklaces with armor. Is one with additional armor better, from an overall mitigation standpoint, from what you've got in the slot now? That's very dependent on the two items, and you can compare them in the lists on the gear forum. (Note that the list places a very high premium on additional stamina! And for good reasons.)
Also keep in mind that armored trinkets, rings, and necklaces will not be multiplied by the Death Knight's armor bonus or by the Druid's bear form armor bonus. (For those classes, when armor is multiplied, the answer to "is armor better?" was a resounding "yes, duh!" Blizz didn't want to make armor standard for those slots, and they didn't want people to have to go for the small number of armor-bearing pieces because they're just THAT MUCH better...)
I wonder if their value is multiplied by the extra 2% from the austere meta gem?
This the tread I originally found on this subject. Now I know this tread is older, and it is before the Wotlk X-pac came out, but it does talk about this "Effective health theory." So with this in mind, does this theory still apply to the game as it is now?