Juul, who is a video game theorist, researcher, and lecturer, writes that the best games, the games we are motivated to continue playing, are games designed to cause the player to fail frequently and sometimes, catastrophically, i.e., to die. Well designed games keep us playing by allowing us to improve and avoid past failures, only to experience future failure, followed by opportunities to once again improve and reach advanced levels of expertise.
There are other game designs that simply reward players for the labor they expend, such as FarmVille, in which players gain new powers by planting and harvesting. The new powers increase the players ability to plant and harvest and expand their "farms." The player is rewarded for labor, not skill acquisition.
Juul includes some interesting psychological and philosophical discussions in his essay, especially those dealing with deception, complicity, and guilt.
All in all, a very interesting book, especially for the game player, who wants to understand how he/she is being manipulated by the game designers.