From WhyWikiWorks:
- Any and all information can be changed or deleted by anyone. Wiki pages represent consensus because it's much easier to delete flames and spam than indulge them. What remains is naturally meaningful.
- Anyone can play. This sounds like a recipe for low signal -- Wiki gets hit by the great unwashed as often as any other site -- but to make an impact on Wiki, you need to generate real content. Anything else will be removed. So anyone can play, but only good players last.
There are also different kinds of consensus, based on different attitudes of the participants. See BlueSky.