Neatness. Your marks are clean and easy to follow, your writing easy to read.
Attention to detail. You naturally notice everything to do with the copy, including spacing, breaking and format, plus the white space and graphics around it.
Confidence. You mark in pen (or even only in pen), rarely in pencil.
As for the text itself, a good editor/proofreader also knows when to query something; when a change might be too drastic, and if it is, to insert a note or query next to it; and the difference between a fair change versus an unfair change, i.e., one that is just based on your personal preference.