Is that that it’s going to be full of rubbish. Wikipedia, which arguably has less authority because contributions can be anonymous, will likely have more authority, since it is more-or-less restricted by its community to one entry per topic.
Another way of looking at this is that on Wikipedia, if you want to contribute to a topic that already has an article, you have no choice but to (try and) improve the existing one. On Google Knol, there’s every incentive to start a new one, never mind if it duplicates existing content, or is worse than an existing one.
Google is good at ranking, and users can rate pages, but Knol is still a mess. You can be sure that many articles will be written primarily to drive traffic to the author’s web site, or to attract Adsense clicks. Wikipedia is not immune to spam; but at least contributors can delete it. All you can do on Knol is to give a spammy article a low rating.
Thinking aloud, what might work is some kind of Slashdot-style filtering. For example, you could have it so that by default Knol searches only show articles which have:
- More than 10 ratings
- An average rating of at least 4
- or are the only article on the subject
or some such; vary the constants according to taste.
Then again, you could have a team of editors (and become Britannica); or enforce one article per topic and become more like Wikipedia.