What if complicated visual design properties like color, contrast, and brightness could all be reduced to one factor? If you're in charge of usability for an interactive product, they can be. Affordance is the term for how perception of an interface influences the opportunity for action.
Affordance theory states that the human visual system is designed to directly perceive action possibilities. In other words, the first thing we see is what we can do in the world. You can apply affordance theory by taking a fresh look at your interface and noting exactly what possible actions it's communicating.
One classic example for the web is hyperlink design. Many designers consider the classic blue underlined link to be garish and dated-looking, but there's no doubt that it stands out and has a strong association with action. Therefore, never underline general text on your site (use boldface or a size change to emphasize text) and don't change text to blue unless you want it to be a link. Conversely, be thoughtful and consistent when you make your links something other than blue and underlined.
Human vision is very good at perceiving the edges of things. Affordance theory explains this because, in a natural environment, a boundary means a change people must react to. Apply this in your design by making sure that functional elements, like buttons, tabs, and form fields, have a strong and consistent edge design. A good perceived affordance means users will immediately know what they can do with your product. They can spend their mental cycles on exploring its value, not deciphering its controls.