Models and Theories in Human-Computer Interaction/Norman's Affordances and Principles of Design
I refer to and think of Norman's affordances and principles of design nearly every day at work and throughout the design process. Its easy to see that when a product is difficult to use it is violating at least one of these principles and these principles contribute to affordance. One of the issues with flat design today is that the designs lack affordance, button styles in Windows 8 don't stand out any more than other elements on the screens and there is nothing to really indicate that they are clickable. The button issue in Windows 8 could also easily be understood by answering the seven stages of action as design aids that Norman mentions. Many people could not tell what actions were possible on the screen, especially with hidden gesture controls and menus. These buttons could have been easier to understand by users by providing better affordance with improved visibility through some kind of visual indicator to make the button seem clickable. Creating a slightly different visual style from the rest of the screen with visual feedback that is consistent across all clickable buttons would also improve the conceptual model provided to the user.
However, I have noticed that Windows 8 doesn't seem to be quite difficult for most people to learn and that is something Norman doesn't really talk about. Even my 62 year old dad has been able to learn it fairly quickly. I posted many sticky notes around his monitor when he purchased a new computer thinking that he would have no idea how to turn it off, but he picked it up right away and had no need for the sticky notes. Does the time it takes to learn the product make a difference even if a design is initially bad according to the principles? In my opinion, everything is learnable which kind of makes everything usable whether its a good experience or not, but our goals as designers should be to make things as quick and easy to learn as possible.