Blizzard has released a rather detailed look into a new feature coming in Cataclysm that we first heard about at BlizzCon 2009 -- the mastery system. The system will "allow players to become better at what makes their chosen talent tree cool or unique." Mastery bonuses are passive and are tied to each talent tree for every class. The more talent points you put in the tree, the more your mastery for that tree will increase. Each talent tree will have three mastery bonuses:

Bonus 1: Increase to damage, healing, or survivability
Bonus 2: Increase to a stat that's relevant to the given talent tree
Bonus 3: A completely unique effect to the given talent tree

The mastery statistic on gear you'll get from 80 to 85 will increase the effect of bonus #3. Mechanically, the point of the mastery system is to remove talents like "6% more damage" and regulate those effects into the entire tree.

The entire announcement from Blizzard after the break. This includes some examples of the mastery bonuses for a few talent trees.