Amidst a boatload of sarcasm and chuckle-worthy examples of perplexing, unintuitive interfaces, UIE founder Jared Spool explained just what makes design seem intuitive--what comprises an easy, breezy experience. Based on users' previous experiences and what they already know, an intuitive design allows for the user to complete a task, no questions asked. Problems arise when questions are asked. An unintuitive design (there's no shortage of them) will spark frustration, pushing the user to question how to use the design rather than focus on what the design intends to do.
Design happens at the "knowledge gap," between what users already know (current knowledge) and what the design needs users to know (target knowledge). When the gap between these two knowledge points is extremely small, or at best, non-existent, the user doesn't perceive complexity or that he/she is learning anything new during an experience. And this is when intuitive design happens. Shazam.