SEOmoz periodically conducts surveys of leading search engine optimizers to determine what they think are the most important ranking factors (http://www.seomoz.org/article/search-ranking-factors). Here is a high-level summary of the top nine results:
Page Level Link Metrics
Domain Level Link Authority Features
Page Level Keyword Usage
Domain Level Keyword Usage
Page Level Social Metrics
Domain Level Brand Metrics
Page Level Keyword Agnostic Features
Page Level Traffic/Query Data
Domain Level Keyword Agnostic Features
Here is a brief look at each of these:
This refers to the links as related to the specific page, such as the number of links, the relevance of the links, and the trust and authority of the links received by the page.
Domain level link authority is based on a cumulative link analysis of all the links to the domain. Factors considered include the number of different domains linking to the site, the trust/authority of those domains, the rate at which new inbound links are added, the relevance of the linking domains, and more.
This describes use of the keyword term/phrase in particular
parts of the HTML code on the page (title element,
alt attributes, etc.).
This refers to how keywords are used in the root or subdomain name, and how impactful that might be on search engine rankings.
Social metrics considered include mentions, ...