In the early '90s Alan Cooper wrote a book called About Face which unfortunately has dropped off the face of the earth as far as its influence on UI design is concerned. One of its many sensible proscriptions was that UI elements that can be interacted with should be visibly distinct from elements that are just there to display information. As a programmer, it drives me insane to have to use any of the modern apps that have completely abandoned this principle - or to have to deal with designers who have literally mocked me for thinking this is important.