Editing, for me, has always been driven by a passion for precision and clarity. Certainly language can be wonderfully vague and evocative, but (putting deconstructionism and critical theory aside, where they probably belong) it is also our best hope for a broad means of communicating clearly and precisely, on topics where that sort of thing matters, to the broadest possible audience. For this reason I love the field of information design. Strunk and White, Steve Krug's Don't Make Me Think, Edward Tufte's Envisioning Information, really good signage, a great headline, effective ad copy, a nice brochure, a solid blog post, a great installation guide, an amazing magazine redesign, and even the small-business process checklists championed by Michael Gerber, all have a great deal in common. Their focus is precision, clarity, and effectiveness in communication: the transfer of information to bring about as predictable a result as possible. The goal is no different when I help an author refine a manuscript, and the same basic techniques apply.
At the moment, most of my freelance income is from book editing. But I have done a fair amount of copywriting, speechwriting, and periodical and package design, as well as some advertising, procedure manuals, information architecture, and the like. I hope to be doing more of each of these in the future. I could easily be wrong but, speaking in general terms, I suspect the core skill set is about the same for all of them. Certainly there are refinements that apply to each specialty, but in none of these endeavors will you be successful unless you can put yourself in the place of the reader and ask, "How can this product communicate as clearly and succintly as possible, on the first read, toward a desired result?" But that skill seems rare. At a minimum, it is rarely exercised well. It's unusual to possess it, it's difficult to hone it, and it's almost never easy to use it.
I am aware, of course, that language is a pure abstraction. In fact, to a limited extent, I would agree with the deconstructionists; as a salaried editor I would frequently offer this maxim when we faced our daily challenges: "There is nothing more difficult than unambiguous human communication." How much more amazing, then, that it can become so effective?