It really bothers me when I see “copyeditor” — one word. Since Day 1, I’ve been far more comfortable with copy editor as two words. Conversely, I have no issue with “copywriter.”
A cursory search suggests that the one-word form is standard at magazines and book publishers, and the two-word form at newspapers. Why the difference, though? The guy who fixes the sink in your company bathroom is a handyman, and so is the guy who fixes it at a restaurant — his title doesn’t magically change to handy man (though he is indeed handy).
Flawed metaphor aside, I’d like to learn more about how and why these conventions develop. It’s one thing for differences to emerge across cultures (the U.K. convention of calling us “sub-editors,” for example) — but when we’re talking about people with similar educational background and expertise doing similar jobs, I don’t know how to feel about such a minute and arbitraryish distinction.