
As I have been thinking about some concepts, an idea came to me that I think might be interesting for any of them. Since we'll be writing the setting as a community aka the Swarm, there will be variation in writing styles even though we will have style guides to direct how to write for the setting. One way that we could capture this as a feature of the setting is to create a persona within the setting and then write parts of the setting from that person's perspective.
As an example for a fantasy setting, contributors could create a sage character and write their submissions from that sage's perspective. In a horror setting, people might create an investigator who has looked into various details within the setting. While I do not think we would want to take this idea to the extreme of having every member of the Swarm create a figure to use for a first person perspective, we could potentially have enough to cover most of the variation. I think it could be a fun approach to creating the setting, too.

Burning Wheel's Test of Wits
Burning Wheel's Test of Wits could even be an interesting mechanic to tie together whether one sage's opinion is the accepted one vs. another. Would people be interested in the setting creation becoming a game unto itself?
Design by Committee
Hey Mike...
It's been my experience that design by committee is one of the worst ways to make a game.
Yes, break up the work so that several people share the workload, but have only one (or two, but no more than two) people directing the final vision and direction of a project. We'll end up with a tighter product that way.
Just my $0.12
peace... Dave
Writing style vs. creative guidance
While I agree that having no guidance on the creative output of the Swarm, I think that competing sages could help with dealing with differences in voice and writing styles. As you said, large creative projects need guidance, but the open style tends to take care of itself. We plan to have part of the winning condition for the winning setting concept author be to commit to help guide and review the contributions to the site.
Also, borrowing from open source software development, contributors will emerge with drive and talent to be asked to take responsibility for reviewing specific areas of the content. For example, someone that has created really strong content for the country of Nixor would be asked to review all the content being submitted for Nixor and lead that part of the setting. This is similar to how Wikipedia functions as well, but it is more open with people taking ownership more independently.