My impression when I first heard about it was that SuperUser would be for (1) PC hardware questions and (2) questions about single-user/client software (basically things you wouldn't find on a server), including operating systems. So I would put forth the suggestion that SuperUser questions should be generally limited to those two categories. (I'm not 100% sure I believe in it, but I'm putting it forth.) Looking at it now, it seems like most of the questions do fall rather cleanly into one or the other.