First, it’s a powerful new tool, but it’s by no means the only tool in the toolbox.

Second, higher health pools and survivability relative to damage means damage output won’t be balanced on the same razor edge in Cataclysm as it is now. When increasing damage by 5% means you knock off 90% of a health pool in five seconds instead of 86%, that’s pretty risky. When it means you knock off 40% of a health pool instead of 38%, that’s not a real big balance changer. Numbers made up, but that’s the general principle.

Third, a greater focus on large-scale PvP will also probably help moderate the effects of changes like this.
This.

Pretty sure PvP gear will have mastery.
And this. (Source)