I think "show, don't tell" is a particular instance of the better (and more widely applicable) rule, "be specific." Showing and not telling forces you to be specific, which is pretty much always a good thing. The problem with SDT arises when it forces the author to spend time on unnecessary details, in effect diluting the focus of the story. We don't (always) need a flashback to a character's childhod to find out why he's afraid of spiders if his arachnophobia only shows up in one scene. Just say, "John hated spiders, so he skipped the trip to the Nature Museum," and get back to telling (and showing) the story about what he does wind up doing.

--macrobat (can't seem to login for some reason)