The Problem with Block
Blocking. Either it's too good, or it's worthless. There's nothing in between.
Block is an unusual mechanic. The idea seems simple, while dodging or parrying an attack makes it do nothing, a block just reduces the the incoming hit. Better shields block more damage. The idea is poorly implemented, though.
You see, in the current implementation, which is going to be corrected for Cataclysm, a block reduces the incoming damage by a set amount. For example, if you block an attack, it might do 1000 less damage. The problem is that it blocks 1000 damage whether the attack was 1500 or 15000. In mathematical parlance, one would say that block, as a function, is non-linear.
This problem puts block in kind of an odd place. Block, as a rule, can either be really good or really terrible. Against an enemy that does a lot of little attacks, block is fantastic. However, most bosses make sporadic attacks for massive damage, and block does next to nothing. Tack onto this the fact that block rating stacks very quickly and doesn’t suffer from diminishing returns, and you end up with the off-tanks for Anub’arak that take no unblocked hits and reduce their incoming damage by 97-100%.
How it’s being fixed
In Cataclysm, block will be percentage based. That is, a block might reduce an attack by 30%, regardless of its size. The result will be that block will suddenly be a good stat for raid tanks, where it had been rather worthless in the past. As an added bonus, blocking will be linear and easier to model for theorycrafters.
What to do in the mean time
Consider your needs when considering block. If you are tanking a lot of mobs, chances are good that block will help. Likewise with dual-wielding enemies. If, however, you are tanking enemies that hit hard (Most raid bosses fall into this category) give block stats a pass in favor of Stamina and real avoidance.
