I haven't quite finished it yet, but it's been an interesting read so far. One of the main premises is that games are fun because we, as humans, have a desire for learning. We want to find patterns in things that might be useful to us in other areas of life. Granted, this need stems from our caveman days, but we still seek 'useful' patterns to this day.
Throughout the book, Koster has his text on one page and amusing cartoons on the facing page. I just got back to a couple of cartoons that I remember seeing when flipping through the book before reading it. The first shows a pit that men are thrown into. They have to climb onto each other in order to escape. Although these are stick men, their postures and thoughts are quite tragic. "My wife..."
The next cartoon has the same stick men, but this time with lines drawn around them. It's Tetris, except the blocks contain desperate stick men. The point is about the 'dressing' of games:
The ethical questions surrounding games as murder simulators, games as misogyny, games as undermining of traditional values, and so on are not aimed at games themselves. They are aimed at the dressing.
Although I don't agree with everything in this book (for instance, I think that good stories matter more than is sometimes implied), it's worth picking up for the $20 or so it costs.