This link was sent to me recently by an SEO contact of mine.
The big three influencing factors in the Google quality score calculation have always been Landing Page Quality, Keyword Relevance (to an adgroups ad's, and matching the keyword / ads to the search term), and Keyword / Display URL / Account CTR. It has long been supposed that CTR was by far the most dominant factor, but Google has recently announced that they are to be placing increased weight on Landing Page Quality, and Keyword Relevance.