I think Mastery is supposed to be the Sam Adams of stats, "Mastery - always a good decision!"

Since the stat itself changes what it does based on your spec, it should always enhance the capabilities of your spec, no matter what it is, while things like haste and crit have a specific function that exists no matter what your spec is. I think this could be a good idea and could iron out a lot of scaling issues - but it depends on Mastery scaling well across all specs.

Currently Mastery is utterly, utterly terrible for SPriests. Sadly, I think it needs to be redesigned, which sucks, because Blizz never likes to redesign anything (unless you're a Pally - man, that class gets reworked every expansion). The current design of Mastery for Shadow is that it boosts two direct-damage spells. Only two. Of these two spells, the one we have now only makes up about 15% of our total DPS in PvE. So, for that stat to be as valuable for Shadow as for, say, an Arcane Mage or an Unholy DK, it needs to boost the power of that one spell a LOT. That would be fine, except for PvP. If that one spell is super-boosted by Mastery you'll have SPriests two- and three-shotting players with 60k HP.

So, since Mastery boosts one spell (with one more available in Cata, which is being designed to be mutually exclusive with the other), there are pretty much two design directions:

1. Mastery sucks for Shadow, but Mind Blast and Mind Spike damage is reasonable in PvP
2. Mastery is good for Shadow, but Mind Blast and Mind Spike damage is OP in PvP

I like the idea of Mastery. I like that Shadow sees the effects of Mastery through an interesting mechanic rather than another flat +damage buff (we have enough of those in our talents). The problem is that it affects one (two come Cata) spell that doesn't account for much of our damage, causing this see-saw feast-or-famine effect.