Content on Wikipedia is published under the GNU Free Documentation License -- when you click any of the "edit" links in a Wikipedia article and begin typing in new content, you're agreeing to submit your content under the terms of the GFDL. When you publish on Knol, on the other hand, your options are to publish under a Creative Commons Attribution 3.0 License (CC-BY), a Creative Commons Attribution-Noncommercial 3.0 License (CC-BY-NC), or a traditional "All Rights Reserved" copyright model. Both the GFDL and the Creative Commons Attribution licenses are popular with content creators who want to give content altruistically "to the world" -- these licenses all allow content to be redistributed freely without modification. The main difference is in the rights that they grant to people who want to create derivative works (modifying or expanding on the original work).
The GFDL is intended as a "viral" license -- if you take a work that is published under the GFDL, and publish a derivative work created from that, your derivative work must also be published under the terms of the GFDL -- that is, also allowing other users to redistribute your work freely and create derivative works from it as well. If a site mirrors Wikipedia articles without including the terms of the GFDL, Wikipedia encourages users to send notifications to those sites pointing out that they're violating Wikipedia's copyright, and if necessary to escalate the matter to their Web hosting provider for noncompliance. You can mirror Wikipedia all you want, and even put put ads all over your mirror site, but you cannot change the terms.
Creative Commons Attribution (CC-BY) licenses, on the other hand, allow users to create derivative works and publish them under almost any terms they want, including an "All rights reserved" model, provided that they give attribution to the author of the original work.
Section 5.5 of Knol's Terms of Service explicitly states that GFDL content cannot be re-published under a CC-BY license