When reading blogs, people only see recent posts and those posts are not significantly revised.

You underrate the amount of people who read blog posts to which they arrive through search engines.

Perhaps only Less Wrong members should be able to view and edit the wiki, and contributors must read a quick start guide and pass a quick test before being allowed to post.

I think something similar is the main reason why Citizendium failed to be an effective competitor to Wikipedia. Barriers to entry.

How much of the target audience wants to improve the world? If most do not, then the wiki would essentially be a net negative on the world.

This assumes that your project won't have a significant effect on the people who participate in it. That's a very bad assumption to make if you want to start a community around self-improvement.