Two days ago my colleague Ta-Nehisi Coates mentioned a lesson-of-the-ages I had passed along when he and I talked recently. Short version: don't write things about people you'd be afraid to tell them in person. This was of course tied to the episode of leaked email from David Weigel, formerly of the Washington Post. Let me say a little more about this, in full wisdom-of-the-ages mode having nothing to do with the current flap, because it took me a while to grasp the whole point.
When it comes to catty remarks involving friends or relatives, this is just a matter of prudence and realism. If a "conscience" is the sneaking sense that somebody may be watching, proper email hygiene is the sense what you write to one person is going to end up... God knows where. Including in front of the person you least want to see it.
The point about intentional, in-writing criticism is a little different. When I was in college I worked on a project for Ralph Nader, long before his days as a presidential candidate. One evening he gave me his own graybeard lecture, from his venerable position as a man who had reached his mid-30s. (A picture from those days at left, though not by me.) He said that a really unattractive personality type was the journalistic bully-coward. That is, the person who breathes absolute fire when sitting at the keyboard, but skulks away nervously if he catches sight of someone he'd so fearlessly denounced from the writer's chair. Yes, this kind of person existed even before the blog age!