A wiki will grow organically as people add new pages and comments, and after a while it can be very difficult for readers to find that "nugget" of information. Try reading the PortlandPatternRepository to find something specific and you will see what I mean. I always find that I'm lost in a labyrinth of tangential discussions.

I suspect you either need to refactor regularly and mercilessly to keep some sort of structure for the reader, or you need add and maintain that structure for it to be useful to the reader: perhaps a table of contents or a keyword index.