SEOmoz, a Search Engine Optimization consultancy, conducted a survey of 37 SEO professionals, asking them to rank several positive and negative factors believed to influence Google's proprietary page ranking algorithm. The influence of each factor was rated from 1 to 5, and the respondents also provided comments. All of the ratings were averaged, and the factors ranked:

Top 10 Positive Factors

1. Keyword Use in Title Tag
2. Anchor Text of Inbound Link
3. Global Link Popularity of Site
4. Age of Site
5. Link Popularity within the Site's Internal Link Structure
6. Topical Relevance of Inbound Links to Site
7. Link Popularity of Site in Topical Community
8. Keyword Use in Body Text
9. Global Link Popularity of Linking Site
10. Rate of New Inbound Links to Site

Top 5 Negative Factors

1. Server is Often Inaccessible to Bots
2. Content Very Similar or Duplicate of Existing Content in the Index
3. External Links to Low Quality/Spam Sites
4. Duplicate Title/Meta Tags on Many Pages
5. Overuse of Targeted Keywords (Stuffing/Spamming)

Most Controversial Factors
(Factors with wide disagreement among the respondents)

1. Manual Authority/Weight Given to Site by Google
2. Relevance of Site's Primary Subject Matter to Query
3. Participation in Link Schemes or Actively Selling Links
4. Duplicate Title/Meta Tags on Many Pages
5. Global Link Popularity of Linking Site
6. Quality of the Document Content  (as measured algorithmically)
7. Domain Extension of Linking Site (edu, gov, com, ca, co.uk, etc)
8. Server is Often Inaccessible to Bots
9. External Links to Low Quality/Spam Sites
10. TLD Extension of Site (edu, gov, us, ca, com, etc)

In addition to providing the above rankings, the article presents all of the factors, their ratings, and the respondents' comments. The factors are organized by general SEO topic: Keyword Use Factors, Page Attributes, Site/Domain Attributes, Inbound Link Attribute, and Negative Crawling/Ranking Attributes.  

SEOmoz also offers a free Beginner's Guide to Search Engine Optimization