I posted an article on my blog about how, at digg, science ended up being the best quality category (largest promotion/submission ratio - http://owenbyrne.com/2008/06/05/yet-another-random-digg-list...). My impression at the time was that there was no money in science - So people post crap constantly in other categories, because that's where the dollars are. There's a lot less money in the promotion of science, hence less sock-puppets, gaming, etc.