I think of all the posts I’ve read on coding horror, this has had the most impact on me in terms of thinking about application design (forgive me that I can’t give you full credit though-- Andy Osram’s paper has had a profound impact too).
I mean it-- it has really got me changing some elements of the software I’m working on.
A few of the comments on your post are welcome as well, in particular the notion that there is a trade-off between time spent accomplishing an objective, and the sense of fun in doing it. As with much else, clearly it’s about finding a balance.
I’m sure it’s no coincidence that you mention all this in light of the badges mechanic present in stack overflow. I think features like that are what keep people coming back. I think the xbox 360 owes much of its success to the achievements system. I almost wonder if it would be even more interesting if on stack overflow all the possible badges were unknown, that rather than working towards them, they were just accidentally discovered… but i digress.
Someone also mentioned wikipedia, but I have to disagree with that comment to some extent. Personally I don’t feel that contributing to wikipedia (along with the scores of forums and mailing lists I trawl through) is a good use of my time. There is almost something of a clique there, and this coupled with what I consider to be a huge learning curve presents an intimidating barrier of entry to me, let alone a casual web surfer.
Of course, some mechanics (as unfortunately, with the themes of several of your articles lately) are more appropriate to web-based software than desktop applications, but nevertheless, this is really good food for thought…