Link to PDF: http://static.googleusercontent.com/external_content/untrusted_dlcp/www.google.com/en/us/insidesearch/howsearchworks/assets/searchqualityevaluatorguidelines.pdf Article: http://www.webpronews.com/google-posts-big-search-quality-rating-guidelines-document-says-its-just-the-cliffs-notes-version-of-the-real-thing-2013-03 The most interesting part of the pdf are: Tips for Recognizing Spam Pages Fake Blogs with PPC Ads Commercial Intent Recognizing Thin Affiliates Recognizing True Merchants
The "Rating factor" of the Google Algorithm it should not make that much importance in the algorithm. I am saying this because it can be very easy exploited. The important factors should be the relevance and quality of the backlinks, the originality of the content, page speed and if the site has a friendly design. Other factors that should also be took in consideration should be the number of minutes/hours that user stays on the site (not on a single page) and the interactivity of the site with the user along with the number of ads placed on the website (the more ads are placed on the site - the more should get the site penalized even if the site owner have added an "external nofollow" to the link/banner ad.