Chris Taylor wrote a piece on Business 2.0 on Why commercial outfits can't get Wikis to work. Among other things it points out the failure of Penguin Books to use a wiki to write a novel. IT failed, even with the assistance from instructors at a creative writing program.
Mr. Taylor concludes that "People need a common focus, a shared obsession, to be productive as a crowd." Wikis need direction. Wikipedia succeeded as first entrant into the market and continues to dominate as the online wiki encyclopedia. Any other wiki needs to be focused on a particular topic of interest. One of my favorites is the Wookiepedia on all things Star Wars.
I think the same concept needs to be carried over to wikis inside the enterprise. The first step for success is for the enterprise to have communities of practice or practice area to organize the "crowd." Each community of practice should have its own wiki. I also think all the changes in the wiki should be distributed to the practice area instead of having to subscribe to changes on a particular wiki page.
A community of practice is more likely to contribute to wiki that is its own rather than firm-wide wiki. I see much more a of sense ownership. The down-side is the possible duplication in wikis for different communities of practice. I think it should be the role of the knowledge management team to monitor the wikis and identify when they are hitting on the same topic. You can then broker who should be the primary repository and link the wikis together using an external URL link rather than an internal wiki link. If the topic is big enough and the groups can't agree, create another wiki for the groups to share and link the community of practice wikis to the new one.
With Wikipedia and its hundreds of thousands of pages and millions of users, a user would be overwhelmed by the changes if they were on the RSS feed for the whole wiki. This scale is unlikely to be true inside an enterprise, with many fewer users and many fewer changes. Also, the flame wars seen in Wikipedia would be unlikely inside the enterprise. Inside the community of practice, mashing all of the changes to the wiki into a single feed makes sure that everyone in the community is aware of everything happening in the wiki.
I have seen some firms set up a single wiki for the whole firm. They generally get poor results.
Of course having multiple wikis means that you need a search tool that indexes all of them and returns the results in a unified manner.