It's basically bonus bonus damage the way I read it. Of course, much of the WoW will treat bonus bonus damage as a measurement of who has the largest member... but in potential reality it would just be best if used as a personal metric to show well you play your spec.

I think this will contribute more towards the old addage of, "bring the player not the class"... or spec (to a teenie tiny degree).

I think I see what ghostcrawler means by, "prematurely." I can't see the Mastery as nerfing the damage over time because you have less mana. As your mana dissipates the damage will become everything it would be with gear and stuff in mind minus the perk provided by the Mastery. To sustain the perk, you would have to figure out how to best use the class mechanics provided (which is what many of you have learned to do pretty well). The reward: ++damage

Generic Example: I see it as if a spell does 1000 damage without the Mastery (base+current gear+buffs+whatever) once the Mastery is applied it's like, "Grats dude, here's an extra extra 200 damage on top of that at full mana, then 150, then 100, then 50, then 20 subsequently... Boost your mana in the process and we'll keep throwing you bonuses!"

You would be a Master of all things Arcane (spec) of course so you should be better than someone who doesn't know how to play the class as efficiently. To me that would highlight skill over cookie cutter gear and specs.

maybe I'm just too optimistic though