Hazards of simplicity
Derek Powazek's Design For Community makes a fascinating observation in passing. Community sites and discussion forums aim to attract posts, to build big discussion threads on hot topics. You might expect that the best way to do this would be to make it easy to post; Powazek finds that it's actually a better idea to make posting difficult, to "bury the Post button" deep in the discussion. For example, in {fray}, you can post only at the end of multi-page stories.
The core problems community sites face are flamewars and graffiti -- casual posts that add nothing but noise, that sap the vitality and spirit of the discussion. Making it hard to post reduces graffiti; only committed people stick around long enough to post. By the time people reach the Post button, they've had a chance to absorb the spirit and the mood of the site.