The latest beta build (12759) included the new mastery system. In addition to the specializations gained when you choose your spec, you also gain access to a mastery bonus specific to your spec. Mastery is a skill that can be trained at level 78, at which point you unlock your mastery bonus and can make use of the mastery rating stat on gear.
The mastery stat will increase the bonus you receive from your spec's mastery (but not the other specialization bonuses). Continue reading after the cut for a the mastery bonuses for every class and spec.