Why are we doing this?

The high levels of tank avoidance players have obtained is making the incoming damage a tank DOES take more ’spiky’ than is healthy for raiding. Ideally, tanks would be receiving a relatively constant stream of damage over time. This allows healers to better plan their healing strategy, broaden their spell options, and simply give more time to react. Tanks could use their cooldowns more reactively. Instead, the current situation is that if we make a hard hitting melee boss and a tank doesn’t avoid two successive swings then the tank could very well be dead in that 1-2 second window. The use of reactive defensive abilities instead becomes a methodically planned affair, healers have to spam their largest heals just in case the huge damage spike happens.

We’ve been trying to do a fair amount to mitigate the effect of high tank avoidance on the encounter side of things during this expansion with faster melee swings, additional melee strikes, dual wielding, narrowing the normal variance of melee swing damage, and various other tricks. There’s a limit to what we can do, however. So to give us a bit of breathing room we?ve implemented Chill of the Throne. Going forward past Icecrown Citadel, we have plans to keep tank avoidance from growing so high again. Source.