I like Susan Lanigan's definition of restraint, and why it can work when it does work: "Writerly restraint is no more or less than affording the reader the courtesy of space to experience the impact of the scene for herself. It’s about pulling back and allowing the reader to infer, rather than constantly poking at her with countless authorial interjections." Yes. And what it isn't, as she notes, is unnecessary distance, the draining of juice and life from a scene, the distrust of emotion.
This is also what I think Walter Kerr was on about in his book How Not to Write a Play, when he lamented, "We are now embarrassed by the dramatic gesture. We do not wish to be thought capable of so gross and unliterary a lapse." And, "In general, we distrust scale nowadays. Certainly we distrust spectacle. We know that the audience yearns for extravagant event; but we are inclined to think of the yearning as one of the least attractive of the audience's characteristics. It is a superficial desire for thrill ... a fairly shoddy form of escape ... [but] I'm not sure that we understand this passion for excitement correctly. It may be a passion for reality, especially that reality which cannot be grasped in any other way."
Sometimes, when we think we are exhibiting proper restraint, we are really just holding back.