I’m skeptical about Knol. While the idea of adding experts to the Wikipedia model sounds great, as with Citizendium it’s not clear if the site will gather the critical mass necessary to make the whole thing worthwhile. But if anyone can take down Wikipedia, I think it would be Google.
Like Wikipedia, Knol (unlike many other Google products, officially it's just "Knol," not "Google Knol") allows anyone to create a page about any topic. By default, new pages are set to use "moderated collaboration," which means anyone can contribute to them but additions only go live after the page's main author or authors allow the contributions to be added.
It's a nice idea. Potentially, it helps solve issues like vandalism yet allows for a broad group of people to contribute. It also is a chief argument in favor of why Google even needs to introduce a tool like Knol, that it is providing what will likely be a robust authoring tool with a unique set of features.