All of it right here http://blue.mmo-champion.com/1/23614404862-mastery-system-preview.html
For the short short version (who likes short shorts? We like short shorts) this is what the mastery stat will give you:
For a slightly longer version: Mastery stat will change depending on what specc you are. How so? well, let's say that you are a priest. You want to spend points in Disc. You spend points in disc. The more points you spend in Disc, the more you will get the following 3 stats as a passive bonus: Healing slight boost to healing powa and Meditation -same as the talent with the same name. Both of these are similar to holy and disc priests (and im pretty sure they will be very similar to every healer in game. But the third. Oh the third. The third passive bonus you will get as a Disc priest is
Absorption – Improves the amount of damage absorbed by spells such as Power Word: Shield and Divine Aegis.Let the drooling commence. But it gets better. The more Mastery points you have on gear and other stuff, the better this 3rd ability gets. So Disc will absorb. Holy will add a HoT to all direct heals, that will grow even more powerful over time and Frost Death knights (how great is this, the preview was for DKs and Priests. Blizz lubs me!!) will get
Runic Power Improves the rate of runic power generated by abilitiesBecause: "While all death knights want runic power, Frost death knights would generally have more runic power than Blood or Unholy death knights (who would receive a different benefit from their respective trees). An Unholy death knight who sub-specs into Frost would still be able to benefit from this bonus, though because they’re investing fewer talent points, they’d benefit to a smaller degree. Mastery on gear would boost this bonus, and no other talent tree would grant it."
Good part over.
Now this is all fine and dandy, if not for the fact that this will very much ruin whatever there was left of creativity in class-speccing. No more, will smart and creative players invent dualwilding DKs with diseaseless speccs that will work. Because mastery-stats will very very much pigeon-hole classes into what is expected from them.
So wave goodbye to the LOL-smite Disc priest.
I know most people laugh at most of these inventions. But the fact is, that most of the classes where never intended to be what they ended up being. They only got build up around these things because crative players used the system to invent new ways of using their class. Disc priest is a fine example of just that. Disc was once a pvp specc for priest. Only not a very good one, since Shadowpriests where way better at melting faces. This was not the intent of the shadow-specc but they where just so gawd-dammed good at it, and fun to play in pvp, that players went shadow. And disc got stuck with nothing. Then Blizz changed Disc around to the damage preventing specc it is today and everyone was happy. I am not 100% sure that this is the established canon on WoW-history. But that was how I experienced the game.
There are a lot of good things to be said about knowing what this and that class and specc can and cannot do, but most of these good things lie in the genre of "comfort". And comfort ends up being staleness which ends up being dying.
But let's hope that the classes will still be able to evolve. Maybe there is a lot to be gained from cross-speccing so that the masteries will have a split bonus? They don't really talk about what happens if you are half n half. Or the longforgotten tri-specc.
Whatever it ends up with, this does spell excitement for Cataclysm. At least for me.