I believe, for the most part, this problem will be avoided if those huge tank-killing hits cannot be blocked/dodged/parried/whatever brewmasters do to mitigate damage w/o using major cooldowns. The big hits that should kill the tanks if they do nothing (or come pretty close to one-shotting them) should require major tanking cooldowns and/or external raid cooldowns... it becomes a breakdown in design if one tank has a huge advantage in a fight over all the others because they don't need to use major cooldowns. This is one of the aspects of SD being just a 40% melee damage reduction while active being a bit too powerful (aka, it was basically a shield wall with no cooldown that would allow druids to not use SI while every other tank may need to use major tanking CDs to survive).
However, from an overall perspective of the direction of druid tanks thus far in MoP, a huge influx of heavy melee damage that can be blocked/absorbed would potentially make bears rely on RNG a bit more than the other tanks. Our armor amount will have to be significantly larger compared to other tanks so that if RNG is not in our favor and we eat every melee hit, we are not instantly roadkill (as you mentioned, generally stopping the damage is a better bet than trying to heal through the damage).