What measures your mastery of your class? Just read a blog post complaining about people on raids who haven't mastered their class. I wonder how can you tell you have mastered your class, because in my case I try my best to learn from each encounter I enter. My view of the mastery is subjective and it's not necessarily equal with someone who has a)played longer, b)mastered more content, c)mastered more classes or d)spends more time reading the off-game resources for tweaking.
How do you rate your own mastery of the class? For me the rating is pretty straightforward: either I can or I cannot. Either my gear/stats are acceptable compared to the requirements, or not. Either I understand the concept/fight/fault or not. In short, I don't have reasonable expectations on my own performance.
After all, as a sort of a perfectionist in nature, I'm my own worst critic and enemy, and I'm very seldom satisfied with myself.
Including my class mastery.