While going for mitigation, such as armor and health, is and has been more effective a long time now, tanks still need to rely on avoidance for much of their survivability. Now imagine that as a tank, your avoidance suddenly drops from 50% to 30%. That is pretty huge and makes you wonder what Blizzard is thinking.
However, once you reflect a bit, you begin to see the wisdom in it. The rattle among many tanks is still going but personally, I am not worried too much over it. The issue mainly stems from the cascading arms race, between tanks and raid bosses. Our gear, both health and avoidance have been steadily climbing, which forces Blizzard to create encounters that hit for more and more damage.
I am certain that many tanks will agree with my own frustration of tanking bosses where you are basically, constantly on the verge of dying. The rotation of two hits and a heal makes your health look like a yo-yo in desperate need of decaf. One slip, or just bad luck, gibs the tank, usually sending what's left of him rolling down hill - in seven pieces.
Now in Icecrown Citadel, Blizzard is attempting to rectify the problem by lowering our avoidance, so they can design more interesting encounters. I am all for this change and it is reassuring to know that this is also the way they will go with Cataclysm; encounters that have more interesting mechanics that affect way more whether you die or not, rather than just raw damage output.
This is not to say Icecrown Citadel will be easy. I imagine it will be the hardest of them all, for obvious reasons. The bosses still swing at the same rate, but will hit a lot more often, with less damage per hit.
However, I am greatly concerned by the fact that lowering avoidance will have a secondary effect on death knights; we rely on avoidance to give us rune strikes. This is our single-greatest threat generating ability. This is why, once we lose aggro, it is hard to get it back because when the boss is no longer swinging at us, we get no rune strikes, and also why we do not have snap aggro.
Reading the latest patch notes for 3.3 might see a fix for this issue, promising a 17% threat increase to rune strikes, to compensate. But still, I worry. Does 17% more threat compensate for the loss of 20% in avoidance? I hope so but will wait for the patch to go live before I count my eggs.