I've played around a lot with different formats, PBWikis, hosted wikis, blogs, etc.
Tying in with what James said, I find wikis tend to grow a lot of stubs (very short articles) and material like this benefits a lot from a Monster Manual - style presentation, i.e. "Here's my cool idea, it's developed enough to require a full page but brief enough to fit on a full page."
That's a very readable, visitor-friendly format, which is important because you want to put the cool stuff in without requiring the reader to invest a lot of time into understanding why your setting is cool.