Someone who's still on the dock
Michael Crichton, as quoted in The Art of Editing, ostensibly talks about editing but really talks about a great many other things:
In my experience of writing, you generally start out with some overall idea that you can see fairly clearly, as if you were standing on a dock and looking at a ship on the ocean. At first you can see the entire ship, but then as you begin work you’re in the boiler room and you can’t see the ship anymore. All you can see are the pipes and the grease and the fittings of the boiler room and, you have to assume, the ship’s exterior. What you really want in an editor is someone who’s still on the dock, who can say, Hi, I’m looking at your ship, and it’s missing a bow, the front mast is crooked, and it looks to me as if your propellers are going to have to be fixed.