#10 breaks multi-turn daily powers that deal damage over multiple rounds. Ie, a power causes ongoing fire damage, it only triggers vulnerability once. If a power lets you deal 1d6 damage as a minor action attack once/turn, you only get to add your enhancement bonus once. Etc.

Or, you need to clarify "per action or power" if that isn't what you mean.

Or you could just make it "per action". The only hole that leaves is warlock double-tapping bonuses with their trap at-wills.