I would pick this for my least favorite truism. My experience is, showing rather than telling does not work. Most readers -- and this includes editors -- don't catch on when I only show. Many years ago, I used to send stories to the wonderful SF editor Terry Carr. He always sent my stories back with a note saying, "This is very well written, but I don't see the point of it." I started to have fantasies of underlining key sentences in red or putting arrows in the margin. The stories sold elsewhere -- to Damon Knight at Orbit and Charles Platt at New Worlds. But Carr never figured out what I was doing.
My second novel, To the Resurrection Station, ends with a fifty page discussion among the characters on 'what has all of this been about?' I wrote the section thinking, "Okay. You want to know the point of this novel. Here it is, in detail." I expected to irritate people with the discussion. But neither editor nor readers complained.
People only think they want to be shown. In fact, they want to be shown and told; and that's what I do these days.
There are many, very compelling kinds of fiction that do far more telling than showing: folk tales, fairy tells, myths and so on. In these, showing is often the icing on the cake or the chocolate chip in the cookie. Don't tell me these stories don't work. Off the top of my head, I think "show, don't tell" is a trusim for modern, bourgeois, realistic fiction, the kind of thing which dominated 19th and 20th century literature in the west. Well, this is the 21st century, and I write science fiction and fantasy. I don't have to show any more than I want to show.