Ghostcrawler responding to a tank (http://forums.worldofwarcraft.com/thread.html?topicId=15199329893&sid=1):
We're not going to promise that every tank has equal difficulty with every fight in the game, particularly those that are designed to be extra challenging. We'll get as close as we can, but at some point unless we give the exact same abilities to every class, some abilities or emphasized stats are just going to work better in some cases than others.
On the other hand, we will avoid very gimmicky fights where you are "supposed to" swap out to a particular class to tank. You won't see a boss in Ulduar that wipes the raid unless Infected Wounds and only Infected Wounds is kept up on him. (Even Razuvious 25 is probably closer than we want to get in the future).
It doesn't help matters that there is only one super challenging encounter in the game now, so "the best tank" tends to get judged by that alone. It would feel different if there were 3 hard fights and different tanks were superior for each of them. (I said "superior" not "impossible without".)
It's awesome that so many players have adopted the "bring the player, not the class" mantra. I think we managed to get that message across. Do remember however that its original context was that after you have the major buffs and debuffs covered that you will have room to bring who you want and generally a lot more flexibility on which classes you need to bring. It doesn't mean that you can put together any kind of contrived raid comp and then point the finger at us when your 12 prot warrior group wipes in Ulduar. :) But that also doesn't mean we are reneging on our promise that you can MT with any of four classes. Just remember things are going to feel the least equitable on the hardest fights.
Now we also need to avoid the situation that Ciderhelm and others describe where one particular tank is superior for learning new content.