Some need a heavy line edit. Some need work on all the transitions. Some need to be coerced into making sense. No two editing jobs are alike. No, I find every author has his or her own issues in the text.
Some problems crop up over and over again, but usually never even in the same way.
At a certain point in my editing career I did realize that some problems crept up with more frequency - for instance, pacing. Most book writers, especially first-time authors, don't know how to pace a whole book. But part of what makes editorial work really exciting is that no two projects are the same. Any liberal arts would be fine. English would give you more of the language.