the first part of PhD researcher and game designer Lucas Blair's study on effective Achievements as interesting as we did, you'll want to check out the recently published second installment over on Gamasutra. While the first article focused on implementing satisfying, compelling carrots in games, this second piece looks more at the more metaphysical components of in-game rewards. For instance, when should Achievements unlock? Once they do, should they be permanent?
Much like the series' first installment, it's definitely worth a read, but take caution: You're going to spend longer than you might like playing its anecdotal Flash game, Achievement Unlocked. Reader beware!