In the late 1870s, scientist and eugenicist Sir Francis Galton developed an image of the prototypical "face of crime" by creating composite photos of men convicted of serious offenses.
Though Galton failed to discover anything abnormal in his composite criminal faces, he did find that the resulting visages were shockingly handsome. (The middle face here is the product of 14 criminals.) Studies have since established that people find prototypical faces—those with average features—to be attractive.
– Maggie Wittlin, Seed Magazine
Back in September, 2006, a paper published in the journal Psychological Science proposed a new explanation for this phenomenon: Prototypical faces are pleasing because they're easy for the brain to process.
"The principle finding is that you like a pattern to the extent that you classify the pattern fast," the study's author and psychologist at the University of California, San Diego Piotr Winkielman said.
On the one hand, this is pretty old-hat to anybody in the design business, and particularly anyone in the interface design business (web or otherwise).
We all learned in UI 101 that (a) a good operative definition of "usability" is that a user doesn't have to think about how to do what she's going to do, (b) that one of the best ways we can accomplish this is give them interface elements that they've already learned how to use.
On the other hand, the Gestalt Laws of Prägnanz provide us with some formal figurations that explain why our brains like puzzles.
Just as doing a bit of physical exercise, mental exercise is not only helpful to us in the long run, but can provide an "adrenaline-rush".
A simple Gestalt 'Figure-Ground' puzzle
So, obviously our designs should be created to take advantage of our user's perceptual fluency both positively (providing familiar UI components) and negatively (using Gestalt and other techniques to provide users with the endorphin-rush of solving a simple visual puzzle).
The really interesting question is whether you can do both of these things at once in a way that preserves the value of each. Now that's a design problem.