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.
Take Search Engine Optimization, for example. Wikipedia has an article that looks decent. Knol has thirty articles; or if you search for SEO, more like seventy. And that’s after just a few days.
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.