Looking at talent trees for all classes, the Systems team felt that players were spending too many points on things like flat damage or healing boosts; talents like Cruelty or Fire and Brimstone are uninteresting but necessary for players due to the bonuses they provide. On top of that, talent trees are bloated with all kinds of talents that are trying to do three things at once, which makes for confusion among players who might not be at the top of the min-maxing game.
What the Systems team wanted to do was make it possible for players to only use their points on "fun" talents -- ones like Body and Soul, Lightning Overload, or Juggernaut -- and make the passive bonuses that used to be in talents your reward for investing points into a tree.
Examples were given for Rogues. A Combat Rogue might see his Mastery bonuses include passive increases to melee damage, hit chance, and armor penetration as he moves down the tree, whereas a Subtlety Rogue might see increases to melee damage, melee haste, and energy regen.
But how does the new Mastery stat tie into this? Ghostcrawler says that the Mastery stat on gear will increase the bonuses you receive from the tree into which you've invested the most talent points. It'll also have other passive bonuses depending on your class and spec -- for example, Mastery will lower the cooldowns on Ret paladin abilities.
This seems like a fantastic change for everyone, and it makes me incredibly excited to think about the fun and interesting talents that'll replace my myriad of spell damage boosting talents. Can't wait for the beta!