1. Niche communities form with lots of quality content
2. This valuable content spurs a spike in viewership, the forum becomes anonymous and no longer a community.
3. Then the quality of content drops dramatically, especially with lots of members just adding banal thoughts or outright karma-whoring. Most content is now in the form of counterpoints which aren't counterpoints, juvenile tit-for-tat fights, massive meme infection and 2-month nostalgia posts.
4. Members who were responsible for quality rapidly abandon the subreddit and the entire subreddit begins to resemble the reddit front page. (I really have no idea how the mods stick it out.)
Unfortunately I have begun to see the same sort of thing happen to HN. HN wised up quickly however and removed visible mod-points which I always felt was responsible for the worst behaviour on reddit. (Slashdot was a bit better because the karma was rationed and the stories curated.)