My daughter, two years old and three feet tall, sees the world differently than I do. She looks up at everything and firmly believes that everything belongs on the floor. She also loves watching music videos on YouTube.
Her use of the YouTube interface is completely different from mine. She can’t really spell so her searches are way off. She likes the Indigo Girls, but can only manage “ind.” Two-year-olds probably aren’t a large market for your software, but everyone is different.
I once worked with a man who was fully red/green color blind, Google is developing an Android interface for the blind, and everyone just thinks differently. Make sure your interfaces are usable by everyone, test them yourself. Try it with a screen reader interfacing or with just your keyboard. Constantly look at everything you make from a new point of view.
Think your interface is very friendly? Try navigating it with your mouse in your non-dominant hand. Do whatever it takes to see things the way someone else does. Even if it means getting down on the floor and looking up once and a while.