I just came across this article that talks about changes to Google’s search results algorithm.
http://www.seomoz.org/blog/beat-google-panda

The five “penalties” listed are:
1. Heavy Template Footprint
2. Empty Content
3. Overlapping and Redundant Articles
4. High Ad Ratio
5. Affiliate Links and Auto Generated Content

But the two significant points from this review are 1) how usable is the site from a human perspective and (more importantly) 2) what is the bounce rate? The bounce rate I’m finding is very important. It looks like search engines are monitoring how long a person is on a site. If the person hits the back button right away the search engine will catch this and change the ranking of this site in the search results.