Mastery

Mastery is a new mechanic and stat introduced in Patch 4.0.1. The effect it has depends on your class and specialization. Mastery has no effect until the Mastery ability is learned from your trainer at level 80. When you learn the ability you will gain an immediate bonus, and you can increase the bonus by equipping gear with the mastery stat on it. The potential of mastery depends on the effect it grants the player. For example, Main Gauche used by Combat Rogues is considered to be rather weak  (especially prior to 4.0.6), while Blood Shield used by Blood Death Knights is considered to be pretty good.