Surprise, delight, and show that we're human
Life isn't all serious. Sprinkle moments of delight and incorporate a sense of play in your application where it makes sense. Brighten up someone's day!
Delight people. Think of ways to lighten up someone's day. Consider the warmth in the design of a smiley in a BBM message, the playfulness in how photos dangle in ScrapBook, or how the paper is torn off when clearing a calculation in the calculator. By adding charm to your application, you show that it was designed by and for people.
This principle doesn't mean that the experience should be whimsical or cartoonish. Make sure that you test your approach with users. What's charming to some might not be charming to others.