I missed this last year, but Derek Powazek of Designing for Community wrote a piece on the design challenges of moderation systems, and the way such systems can actually encourage users to game them. He goes into some detail about the way Slashdot's numbered karma system led to rampant karma-whoring, where users treated slashdot not as a forum but as a kind of social pinball machine, where the goal was simply to make the karma number go up. (His understanding of the system's weakness pre-figured slashdot's eventual move to named karma categories -- Good, Excellent, etc) He also contrasts the positive and negative aspects of slashdot with MetaFilter and Kuro5hin, and ends with an assertion about the need for social secrets in moderation systems (something I've been wondering about vis-a-vis Nutch):