I suspect my reactions to this book may be different than the majority of readers, as I assume most readers will be dedicated gamers or those who is close to someone who is a serious gamer and want to know more about it.*
First the positives about this book.
I appreciate than Jane McGonigal brings a fresh perspective and intelligence to looking at games. She draws from research in a variety of quarters which often are elucidating. For example, despite reading many smart popularized books on behavioral and cognitive brain issues, I hadn't read anything about why teasing (or self-deprecating humor) is an effective way of building camaraderie. And of course, she does a good job building a long and detailed case for what games have to offer. Since that has been discussed in detail in many other reviews, I won't say much about it.
Sometimes I felt there was too much detail, even after a simple point was made. I listened to the audio book, so I can't cite chapter and verse, but one example I remember is that she mentioned something about rock stars and reeled off a long list of stars, finishing with the line "to name a few." Well, actually, you named way more than few. I know this may sound like nitpicking, but it was indicative of an overall sense of wishing she'd make her point and move on more rather than give LOTS of examples. My lack of interest in specifics about particular games may be coming into play. Plus the fact when listening to an audio book, you can't skim effectively. So take that criticism in context to your own interests and form of "reading" this book.
Another issue I had, and one that seems to be a problem with so many books, is that she is so focused on building a case (in this case, how great games are) that she seems unwilling to seriously consider counter arguments. As if entertaining such thoughts would be dangerous to her whole thesis.
The biggest unconsidered argument and I suppose my biggest problem with the book (I'm ready to duck from gamers who are likely to be mad at me), is that she doesn't look at the deep problem of people turning to games because they are disappointed with reality. While I'm very interested in her desire to make human activities more engaging by borrowing from what is engaging about games, she doesn't address that turning to games instead of working on accepting reality is, in the long run, damaging or counter-productive--to improving our long term psycho-spiritual health. Watching lots of TV can feel good too, especially in the short run, but it tends to be a way to escape facing our reality and not moving us toward working on accepting the difficulties of being human--which I believe is the real mission/journey for all of us. I suppose that is the ultimate game.
*Well, I should qualify that I love games, but more of the board or card variety (esp. card games) than video games (though I suspect based on a minor Pac-Man addiction I had for a brief while in college, I'd probably get way into them if I tried).