It has some really great thoughts on what exactly games are, and on what makes them good. If you have any interest in games, you should read (or re-read) it.
I'll also add the bullet points from a couple posters and talks I've given on why serious games are good for learning:
- Soft failure: we learn by making mistakes. In a game, you can make mistakes without negative consequences.
- Topsight: exploring the dynamics of a system by playing with it gives you a holistic understanding of it that is hard to acquire otherwise.
- Engagement: experiencing a subject in a game is more interesting and involving (i.e., fun) than being told about it in a presentation or text.
- Practicing Surprise: games can surprise you, and let you practice dealing with the unexpected.
When educational games use these strengths, they can be awesome. When educational games suck (and sadly, they so very frequently do), it's generally because they have failed on one or more of these points. Heck, sometimes regular games fail on these points (or something related) and it causes them to suck.