With Icecrown Citadel's unpopular Chill of the Throne debuff, or as we like to call it, "Getting out of Dodge," tanks have had to re-prioritize in terms of avoidance and mitigation stats to the point where tier 10 gear might not be as desirable as Blizzard intended. Thankfully, they appear to have realized this, and change is coming! To tier 10 tank gear, anyway. Blizzard CM Zarhym posted a note on the official forums announcing their plans for said gear:
"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."