I wouldn't say a degree in English (or anything, for that matter) is a must--some of the most stilted, unreadable writing, after all, is in doctoral theses--but a love of literature, language, and research into a variety of topics is most likely a given. Two of my favorites, PN Elrod and Elizabeth Peters, have degrees in theatre and archaeology, respectively. And as far as we know, Shakespeare never even went to university, but managed to do pretty well in his chosen field.