Sometimes I browse the iPhone App Store looking for a game to play. Games that are considered tend to be more simple. Angry Birds, and now Tiny Wings, is incredibly popular; I believe their simplicity results in their popularity. Open the game at any time, play a few rounds, and be done.
Having been an avid gamer in the past I don’t like games that draw me back in. I have other priorities with my time and don’t want to spend hours playing games. The interface on both of these games is simple and easy to understand. There aren’t complicated functions and keystrokes, just swipe or press your finger. Simplicity rules.