Not all editors do all the things below. This isn’t an all-inclusive list. (I’m not even touching line- and copyediting.) But it’s a start.
Patricia C. Wrede has a lovely post touching on some of this as well. Patricia C. Wrede covers pretty much every important writing topic on her blog, and frankly, your time is better spent over there than over here.
An editor is often responsible for multiple products or multiple stories within a single product. It’s the editor’s responsibility to consider how each story fits into the grand scheme of publication. Does each contribute something unique, or are two stories–perhaps both brilliant in their own right–too similar?
An editor working with an established franchise must ask related questions–questions as broad as “does this story enrich the themes at the heart of our franchise?” and as specific as “does this story match up in continuity with stories we’ve published before (and does that matter?)”
An editor thinks about audience expectations. An editor at a company with a reputation for intimate, personal stories reminds writers of the alienating potential of a high-stakes action story (and vice-versa). An editor remembers that readers and gamers and so forth are busy people who, by definition, don’t care as much about a story as its writer–and can’t be written off as “lazy” or “dense” because they don’t understand the writer’s convoluted plot or pick up on her obscure themes.
An editor is pragmatic, watchful of schedules and deadlines, accommodating where he can be but always mindful that any one person’s lateness may affect many other people (artists, printers, etc.)
An editor sees problems in stories that the writer doesn’t notice… but that the audience will. Unlike the audience, the editor knows how to work with the writer to identify and eliminate the root of the problems and not merely the symptoms.
Preferably, the editor also catches these problems early. This is why editors like outlines.
(Say… shouldn’t our theoretical writer be skilled enough to avoid problems in the first place? Yes, she should–in the same way a computer programmer should be skilled enough not to write buggy code. But programmers still do code reviews and need QA, and no one holds that against them.)
An editor has an understanding of craft and techniques that can be applied to solve problems in a story’s construction. Just as important, an editor knows how to help a writer use the skills that writer already possesses to fix problems; an editor who insists (even correctly) that a saw is the best tool for the job when the writer only knows how to use a hammer must either help that writer learn how to saw or find a way to help make that hammer serve.
An editor teaches a writer new techniques and new tools, when opportunity and time allow.
An editor has preferences and passions that inform his selections for publication, the authors he works with, and his editorial guidance. This doesn’t mean an editor ignores the preferences of authors and audiences in favor of his own, but great editors cultivate a unique stable of writers and raise them to heights they could not otherwise reach. An editor is like a museum curator, arranging exhibits and selecting artifacts for display in a unique and visionary way.
An editor recognizes that there are well-crafted, high-quality stories that she may not be particularly fond of–stories that fall outside the bounds of the preferences and passions mentioned above. She knows that these stories still have an audience and are still worth publishing (on both an artistic and financial level–and an editor should always be mindful of financial viability).
Speaking of which, an editor makes sure to publish stories popular and profitable enough to keep the company running. Otherwise, he won’t be an editor very long.
Ideally, an editor knows other editors who can take on the projects worth publishing that she doesn’t have a passion for. Alas, we don’t always live in an ideal world, and an editor must also know how to polish and improve projects she doesn’t particularly like. As mentioned above, the editor must be thinking about audience expectations–she may not like the project, but someone out there will, and that’s who the editor is working for.
An editor shields the rest of the company–art, marketing, and so on–when a writer gets unbearably pretentious and argumentative. Because someone has to do it.
An editor knows how to hire writers. Professional writers. You’d think other writers would know how to do this, and many of them do, but not all.
An editor helps a writer tell the story he means to tell–even when the writer doesn’t know how, or even what that story really is.
An editor lets a writer focus her efforts on writing a story she believes in. An editor makes sure that story can live and thrive beyond the writer’s desk.