As a copy editor, your job is to enforce the rules of grammar, spelling, and style, and do so with enough conviction and authority that you don’t have to provide a remedial grammar lesson with every correction. You also have to keep track of the logic of the piece and be mindful of how your changes will cascade. Is a name spelled the same way each time? If you replace a pronoun with its referent, or vice versa, will you have to swap them out in the next clause or sentence to avoid unnecessary repetition? If you change the tense or the number of a verb in one place, will you have to update it in every subsequent instance? (See also: continuity supervisor.)
Not as well known is your obligation to read for sense: to make sure the writer is, in fact, saying what he or she means. Because there are usually deadlines that preclude a leisurely meeting with the writer, there’s fair amount of mind-reading involved. You must become very good at intuiting both what the writer intends to say and what the reader will understand. To be a good copy editor, then, you do not just need to be anal, you also need a lot of empathy.
One of Cary Tennis’s hallmarks is that he devotes his entire column to a single query, so he can go deep. He teases out the nuances of both the situation and its aftermath, not just saying what needs to be said, but anticipating, more than any other advice columnist, how it will be received.
A detail-oriented authoritarian empath could easily have become a judgmental bully, but Cary is a patient and gentle adviser. For all I know, he's an asshole in real life, but on paper—er, online—he’s much kinder than I could ever be.