Robert Towne spells it out

In rewriting what you have to be able to do is read a piece of material, say what’s wrong with it, know how to say what’s right with it, and then be able to do it yourself.

That’s really what it comes down to. Some people say what’s wrong with something, some people can even say what’s right with it, and some people can do all three.

But, you know, the more things that are required, the fewer people can do it.