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!
This obvious part of the message was: think about how you write. But the other part was: think about how you are. Nader very definitely did not mean that you should never criticize people harshly or in writing. He meant (to shift ethnic registers -- his background is Lebanese) that you should be a mensch about it. Minimize the gap between "to your face" versus "behind your back" discourse. Be willing to encounter people you've criticized.
No one fully eliminates the gap. Not many normal people enjoy the face-to-face meetings with those they've said harsh things about -- Andrew Sullivan's account of House of Commons-style verbal-combat kabuki notwithstanding. But I think both halves of that Nader recommendation are useful. Write as if you might run into the person afterwards. And when you run into people, be comfortable owning up to what you've said and where you disagree.*
* This model doesn't hold me back from criticizing people -- with one exception. I've reached the point where I won't write a book review unless I like the book. If I think a book is bad, I'll just say: Sorry, I don't want to write about this one. Creating any real book is hard and represents a significant effort and gamble on the author's part. Unless it's a dangerous piece of agitprop, or unless it's being widely praised for reasons I consider crazy, it's just not worth it to me to go through the exercise of saying how and why I think it falls short. Enough other people will do that; and besides, authors never forgive bad reviews. On the other hand, it's always worth criticizing public officials. That's part of the basic bargain in their line of work, and the consequences of what they do matter.
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