Pretty straight forward question. Mastery for certain specializations are very simple, like Frost Death Knights "increase Frost damage done" and Retribution Paladin "increase Holy damage done". Boring. Not much thought into it other than flat damage increase, being that they are DPS roles.
Even Holy Priest's Mastery is a HoT on top of your heal. Also boring, and redundant.
Demonology Warlock and Beast Mastery Hunters have quite literally the same Mastery, just different names.
Now I know these examples fit the theme/kit of the specialization, but then you have interesting and unique Mastery for Windwalker Monk (deal more damage when your abilities are not a repeat of the previous ability) effects your play style and even MM Hunter (casting range increased) and Frost Mage (creating of icicles to use Ice Lance at certain opportune times) where your Mastery can effect your play style.
As a concept, I like the Mastery stat. I also personally feel like the Mastery mechanics for Frost Death Knight, Unholy Death Knight, Retribution Paladin, Demo Warlock, BM Hunter, Havoc Demon Hunter, Holy Priest, pretty much all Rogue specs... should be redesigned.
What do you think? Leave all of them the way they are? Make others more interesting?