I'm both a writer and an editor, and I like to think I do both well--but not at the same time. When I'm writing, I get too close to the work to see the things that should be changed. When I'm editing, I tend to stick mostly to proofing, minor changes and gentle suggestions for improvement rather than doing major re-writes (unless, of course, I'm specifically directed by a client to tear it apart).
I suppose this is common among people who wear more than one freelancing hat, but it is a limitation I wish I could overcome. Becoming more ruthless with the red pen on somebody else's work is probably going to be an easier hurdle than gaining sufficient detachment from my own work to edit it thoroughly. Any suggestions?