I'll also add that high avoidance plays into this problem too. When tanks can dodge or parry 50% or more of attacks (to pick a random number) then bosses have to hit twice as hard when they do connect.
GC also quoted someone, part of what they said was this: "The fights are structured so that wether the tank actually needs it or not, a heal needs to be on the way just in case you get hit."
Avoidance is out of control. Blizzard recognized this with Sunwell, but they either didn't realize the full extent of the problem or they did not take actions to fix it. Avoidance is too high but also not useful enough. As the poster stated, the heal has to be cast anyway. Avoidance serves to only hurt players.
Since the heal has to be cast anyway, avoidance just makes it an overheal.
Avoidance is just another stat. Since it cannot be relied on by the tank or the healer, it is taking away from other stats which would add mitigation or aggro; such as strength or agility. I'll be honest, I feel kinda silly calling agi a tanking stat, but it does add armor and crit, so it fits.
Hitting harder = harder?
Yes. No. Maybe?
Avoiding half the attacks but getting hit twice as hard averages out with no avoidance and regular damage. However avoidance and the boss damage compensation just means increased risk (hitting much harder) without much benefit (since the healers still need to heal, they can't gamble on avoidance).
So what's my brilliant suggestion?
Nerf avoidance heavily and boss damage as well. Cut avoidance from routinely breaking 50% to more like 20%. Shift the stats towards more damage/mitigation such as agility or strength. I don't want to see the avoidance gain from rating reduced, since I want avoidance to be worth the cost, but instead the devs should intentionally add less avoidance to gear.
This will strengthen block tanks significantly. Block will become a stronger source of mitigation. Sure, a 2k block isn't that great when a boss hits for 20k, but what if it hits for 10k? That's not too shabby. It would be even higher if there was more strength/BV and less avoidance.
This would make gear much more interesting. If aggro was nerfed so tanks had to actually fight a bit for aggro, they'd look more towards aggro stats. Heavily nerfed boss damage would also make effective health a far less important concept, meaning that stam stacking would be silly.
Non-block tanks would have around 20-25% avoidance while block tanks would be much lower, 5-10%. The goal is that they'll still avoid more, but no one gets so high that Blizzard has to go back to hugely scaling boss damage to compensate. Overall though, tanks would take more damage than currently.