The Future of Games from a Design Perspective on GameSpy offers a recap of a discussion of game design at the DICE Summit given by famed designer Peter Molyneux. Along the way, he offers a simple summary of how good game design is suited to simple summaries:

Molyneux pointed out that whenever you try to tell a friend about a great book or a great movie, they often ask "What's it about?" A game is no different. Successful games can be summed up in one line, no matter how wild or unconventional the final gameplay or technology behind it. For example, Populous is summed up with the idea: "You are a God." "Build your own theme park" is the concept behind Molyneux's Theme Park. The concept is something people can easily grab on to, easily understand -- they know they want to play the game before they even see it. "This is the lynchpin of developing a game," Molyneux explained.