Good portraits fail. They fail depending on, of course, what you're trying to express or see, but they fail because they show you exactly what famous people look like, or anybody looks like. A line, line, like a duplication even. People have made big careers out of simply photographing people under the same light, ad nauseam.
I don't care what somebody's face looks like, but I care about who they are, what they represent, their energy. How do you express that? I have no idea.