Greene is able to achieve a wonderful state of equilibrium in his books, in my opinion; it's not excessively layman in that it leaves those familiar with the material without reverance, and it's casual enough that you can almost read it like many would a novel. It doesn't read like Feynman, but it doesn't read like Dan Brown either.
Of course, the above is my opinion of The Elegant Universe, so it's possible his new book abberrated from this preferred (by me) state of equilibrum.