Sure, plenty of my colleagues wear flip-flops to work and it doesn't bother me. And it seems that we're more or less in agreement that resting feet (shod or not) on armrests and bulkheads is rude. But why are people bothered by, say, witnessing a complete stranger walk into an airplane lav barefoot? My guess is it's a sympathetic reaction of sorts. Person A would feel incredibly skeeved out being barefoot in a lav (or perhaps even in an airplane aisle), person A sees person B walk into a lav barefoot, so person A feels personally disgusted even though it's someone else's feet coming in contact with unmentionable substances.

Anyway, think I've reached my armchair psychology quota for the day