Actually, they're quite different:
- Wikipedia is team-oriented, with multiple community members contributing to a collaborative article on each topic.
- Knol is author-oriented, with individual writers contributing many different (and competing) articles on each topic.
If I were looking up widget disease or somewhereville for an overview of that topic, I'd use Wikipedia. If I wanted to browse a whole bunch of different articles on widget disease or somewhereville, with the confidence that at least some of those articles are written by academics or other experts, I might spend some time clicking around Knol.
- In terms of quality control, Wikipedia relies on community members, and Knol will rely on reader ratings.
- Unlike Wikipedia, which is an editorial free-for-all, Knol should be appealing to academics who want to disseminate information or establish credentials with articles that carry their bylines and are fully under their control. (On the other hand, it remains to be seen if academics and experts will want to share a stage with every Tom, Dick, and Harry who's trying to sell or promote something to to make a quick AdSense buck.)
Finally, those who have felt threatened by Wikipedia will also feel threatened by Knol, but history shows that individuals who create valuable, authoritative sites on their topics can compete very well with the megasites (including Wikipedia) in Google Search.