It takes a genius to get a grad degree in a language you were raised your whole life in. 
Well, in all fairness, and speaking as an assclown with a BA and MA in English myself, the concentrations vary, but it's typically between a literature focus or linguistics. It's *much* more involved than just learning to speak and write in our native language; it entails things like dissecting literature, such that you can teach others how to break that shit down to fully understand what an author is trying to relate to the reader, etc.
Just to give you an example, consider Orwell's _Animal Farm_. It's a thinly veiled story about the history of the nascent USSR. English instructors will have students read material like that and encourage them to look for such deeper meaning. Sometimes it's transparent; others, not so much.
But another key duty to any English teacher is, well, to teach how to write *well*. That's not as easy as it sounds!
Take a Matt C. post...he typically repeats himself many times over. Others will try to multiply their adjectives to extreme degrees and/or use fifty words when ten would communicate the same information just as thoroughly. What works better: "Jesus wept," or "The man known as Jesus was so upset that he cried"?
The former, obviously.
All that said, Prince might've earned his MA, but I don't give a flip. When he was gassed, he was a royal prick online. I don't wish further illness on him, but the day I pity him is the day I'm stiff in a cheap casket.
