A few years ago I had developed a theory of game playing and low pressure social group interaction which starts at a similar place as Koster's. I was able to take that starting point about play and patterns and produce empirically testable hypotheses with formal mathematical models of what is happening during play.

And then I stopped working on it because I couldn't seem to get across the concept that learning and fun might be related well enough. Now that I've had a chance to read his book, I might have to reconsider.