I don't like being mean. However, after a certain amount of beta testing for each expansion and the slew of questionable experiences that that tends to bring, I've noticed that I will eventually write something that veers into the realm known as "unpleasant." For Wrath of the Lich King, it was over the raft of players rolling death knights who had never played a melee class before and were making life hell for their groups. (Although it turns out that "Most of you are awful" was a less controversial opinion than anticipated.) For Cataclysm, it was to Blizzard over the fact that resto druids couldn't hit anything except Nourish or Lifebloom without gasping for mana, and I'm pretty sure our subsequent experience in tier 11 bore me out there. (In short: It sucked.)
In the Mists of Pandaria beta, I figured I'd hit the old Cataclysm heroics first for a more forgiving playground with the new skills and talents, and lots of other players had the same idea. Consequently, I've run into a pretty wide variety of tanks and healers with different gear and skill sets, and something has become horribly obvious after running a slew of 5-mans. Nor can I blame unfamiliarity with the content for poor performance, including my own. We know these dungeons. The only new variable is whatever's in your spellbook that wasn't there before.
This is where things get mean, and I apologize:
If you are a bad tank and aren't interested in getting better, Mists of Pandaria might be a good time to find something else to do.