On HN with its dang moderation that is often praised, I have noticed a tendency to ascribe Slashdot's fall to poor moderation. That wasn't actually how things happened, though.

Slashdot's decline started in the years around 2004 because of a site redesign that was taken very badly by the community, and the sale of the site to new owners who began to make all kinds of annoying changes with a view to monetizing. A lot of longtime users bailed out. It was only due to this loss of the productive participants that the site seemed to be taken over by low-quality posting.