At the beginning of each semester, I ask the students in my editing class to fill out a form with a few personal details: hometown, major, and the like. And I ask the title of the last book read for amusement—that is, not assigned for a class.
Bitter experience has taught me to add, "If you can't think in thirty seconds of a book you read under your own steam, leave that blank." Someone always does.
That is my first indication that a student shows little or no promise as an editor. I don't know how anyone who has not engaged in a wide range of reading can hope to edit effectively.
Reading widely is how one accumulates an indispensable fund of general knowledge. Knowing grammar, usage, and house style is essential in the work, but an editor has to have enough furniture upstairs to spot bad information. You do not have to be a polymath to edit (though that would help), but you have to know a little about a great many things and a lot about a few things. Among my colleagues over the years, I've identified who knows about history, who knows about science, whom to ask about sports, who follows popular culture, who is acquainted with the local lore.
Accumulating general knowledge is only part of what an editor gains from the enterprise of reading. Wide, omnivorous reading is how one discovers the possibilities in writing, the different kinds of excellence. Reading John McPhee's books over the years has shown me how to prune to the essentials of complex matters and explain them with clarity. Reading Robert A. Caro's books on Lyndon Johnson has shown me how facts of a complicated narrative can be marshaled and ordered. Fiction, too: Flannery O'Connor's toughness and Eudora Welty's gracefulness mark the possibilities of the Southern vernacular. Those four and scores more encountered over six decades of assiduous reading help me to make informed judgments. How can you expect to repair bad prose if you aren't thoroughly acquainted with the best?.
Yes, to be a writer, you must first be a reader. And to be an editor, you must first be an avid reader. You have to hear all the voices to know which one is appropriate for your writer, your subject, your publication, your reader.
You think you want to be an editor? Hit the books.