From the About page:

Less Wrong is a partially moderated community blog that allows general authors to contribute posts as well as comments. Users vote posts and comments up and down (with code based on Reddit's open source). "Promoted" posts (appearing on the front page) are chosen by the editors on the basis of substantive new content, clear argument, good writing, popularity, and importance.

We suggest submitting links with a short description. Recommended books should have longer descriptions. Links will not be promoted unless they are truly excellent - the "promoted" posts are intended as a filtered stream for the casual/busy reader.

As far as I can tell, most people vote along the lines of the promotion criteria.