Robert Giroux on editing:
“Many elements go into the regular editor’s making, starting with the accidents of background and schooling. But there are three qualities that cannot be taught, and without which a good editor cannot function— judgment, taste and empathy. Judgment is the ability to evaluate a manuscript and its author. Taste is subjective and difficult to define, but we all recognize it when we encounter it. Empathy is the capacity not only to perceive what the author’s aims are, but to help in achieving their realization to the fullest extent. The Pygmalion role, a desire to reshape the writer in the editor’s image, is anathema. . . . To the three basic qualities of judgment, taste, and empathy, all the rest will be added by time and experience, except (let me quickly add) that a little luck never hurts.”
That passage from his lecture “The Education on an Editor” is quoted in the introduction to his Paris Review interview of 2000, which is worth your while.