As a DPS, the only concern you have is churning out rad numbers and showing the haters where you come from. To do this, you look to your gear and figure out what your GOD STAT is. Right now, those are mostly Strength, Spellpower, or Attack Power. Let's say your a shadow priest, because you're a classy gent that appreciates the finer things in life, so you love spellpower. You gem for it at every turn, you roll around in it, you want it more than anything else point for point.
Other stats are pretty much defined in light of the GOD STAT. Maybe they're a cap stat like Hit, in which case you have to do an elaborate dance to make sure it stays capped, but not much more. It's better than the GOD STAT up to a specific point, then turns to garbage. That's not interesting, that's just annoying and finicky.
But what of Crit and Haste? Spriests love those too! Yes, but not as much as spellpower; in fact, I know there exact conversion rates by heart (.76 and .98 respectively). I love Haste exactly 98% as much as I love spellpower. When I see new gear, I go through an elaborate arithmetic ritual of turning all the stats into spellpower-equivalent numbers, adding them up, and seeing if that matches my current piece (or going on shadowpriest.com and looking it up). So if it's not a GOD STAT, nor a cap stat, it must be a conversion stat. Which isn't interesting; it's in fact identical to just having the GOD STAT, but with more legwork.
Speaking of legwork, there's the fourth kind of stat: insane horseshit stat. Spriests, as a member of the spellcasting master race, don't have to deal with this, as I'm pretty much just talking about armor pen. If you don't have a master's degree in Especially Difficult Math, odds are you don't really know how good AP is for you; there're spreadsheets that will tell you though. But bizarre trinkets will change all of this if you have them, so back to the spreadsheet! AP is pretty unique in its insane horseshit as a gear stat, which is why Blizz is getting rid of it. Good move.
The question is, where will mastery fall? Not a GOD STAT I would venture to guess (at least not for a significant length of time), since Blizz wants to emphasize the 5 main stats. The mastery bonuses seem pretty linear, so a cap is unlikely for most classes, but I wouldn't rule it out. Maybe Frost DKs only need so much RP boost before it's just pointless. And certainly not insane horseshit: that's what they're trying to get away from.
For most DPS classes, I would wager it's a conversion stat. Hooray, another number to put into my alchemical gear-goodness formula. Hooray, my (.76*Crit+.98*Haste+.59*Spirit+.22*Int+Spellpower) turns into (.76*Crit+.98*Haste+.59*Mastery+Int). I'M HAVING SO MUCH MORE FUN THIS WAY.
Maybe for healers and tanks it will be interesting to balance, much like healers might juggle mp5, haste, and raw throughput. And I can't say it won't bring more depth into the differences between classes. I'm not complaining here, I'm just not seeing how Mastery will make gearing any more interesting as a DPS. It will just come down turning all stats into one number, so you know Item X is +840 GOOD. Your item is only +790 GOOD, so that's an upgrade. That's all.
Talent tree mastery is pretty smart, though.