I joined SO early and it had a "gamified" interface that I actually found fun. Putting in effort and such I able to slowly gain karma.

The problem was as the site scaled, the competition to answer a given question became more and more intense and that made it miserable. I left at that point but I think a lot people stayed with dynamic that was extremely unhealthy. (and the quality of accepted questions declined also).

With all this, the moderation criteria didn't have to directly change, it just had to fail to deal with the effects that were happening.