GoogleScore = KW Usage Score + Domain Strength + Inbound Link Score + User Data + Content Quality Score + (Manual Boosts) - (Automated & Manual Penalties)

Keyword Usage Factors:
Keywords in title tag and Header tags
Keywords in document text
Keywords in internal links pointing to the page
Keywords in domain and/or URL

Domain Strength

Registration history and Domain age
Strength of links pointing to the domain
Topical neighborhood of domain based on inlinks & outlinks

Inbound Link Score

Age of links
Quality of domains sending links and pages sending links
Anchor text of links
Link quantity/weight metric (Pagerank or a variation)
Subject matter of linking pages/sites

User Data
Historical CTR to page in SERPs
Time users spend on page
Search requests for URL/domain
Historical visits/use of URL/domain by users GG can monitor (toolbar, wifi, analytics, etc.)

Content Quality Score

Potentially given by hand for popular queries/pages
Content should be readability and text quality.