Armor is the new Black (T-10 Set Change Inc).

This just in from the official forums, there are changes incoming to the plate armor tier-10 tank sets. From Zarhym:

"We recognize that many plate tanks are making their gear choices based on the amount of armor they're provided, as this is currently the most preferred stat. In order to make the tier-10 plate tanking sets more desirable, we will be adjusting the stats on the gloves and chest pieces in order to inflate the amount of armor they provide in the next minor patch. This will apply to all item levels of the tier-10 death knight, warrior, and paladin tanking gloves and chest pieces."

Why are they doing this? Chill of the Throne is why. With the reduction in dodge, tanks have turned back to focusing on mitigation; that is, stacking on more armor to reduce the hits that they are not more likely to take. With the higher iLvl gear available in ICC, tanks don't have to sacrifice all of their avoidance to do this. Some, yes, but not all.

To help illustrate this point, here are two tank items: the Warrior Tier-10 Gloves and the Frost Emblem Gloves. The tier set grants more total avoidance (and the added benefit of hit), but the Emblem gloves have 658 more armor. To put this into perspective, let's check out my warrior. Unbuffed, at rest, I have 26,829 armor. This puts me (and I forgive me if my math is off) at 61.7% damage reduction from armor. I don't have the Tier-10 gloves, but hopefully this will illustrate the effect of adding more armor to gear. My damage reduction at 27,487 armor works out to 62.3%. That's roughly 0.6% more damage reduction. Doesn't seem like a lot (if you were getting hit for 20,000, you'd now be getting hit for 19,880). Looking through the available ICC "armor" items, though, you'll find that you can add 3,746 armor, and another 465 with armor enchants. Add in a ring and neck piece, and the most amazing trinket a tank has ever seen, and the grand total comes to 7,683 more armor than without armor stacking.

Let's take a look at the impact this will have. Starting with my Warrior's 26,829 armor, let's add the bonus armor (7,683) on top: 34,512. Running this back through the damage reduction calculations, we get 67.5% reduction from armor, a gain of 5.8% damage reduction. That 20,000 damage hit is now down to 18,840, a reduction of 1,160 damage. That's almost as much as blocking the attack outright. Add on to this the extra socket that most "bonus armor" pieces have and you'll see that they become very powerful items

The ultimate goal with armor is to reduce the damage of attacks that are not avoided. In ICC, landed hits are more common because of the dodge debuff. We don't know yet exactly what will change with the tier pieces, but I would expect to see them become more well-rounded, so don't plan on them having as much armor as the "bonus armor" pieces. Hopefully, though, it will make tanks desire their tier pieces as much as they have been the non-set pieces.