The big question is always, "What factors does Google weigh?" What matters to Google?
Of course, they're not publishing the comprehensive list with weights and details. And there's really no way to know what factors Google is actually looking at.
But a good start is to work out what Google can actually see and then figure out might be significant.
So what can Google see?
Here's my approach at a comprehensive list:
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Domains
- length of domain name
- keywords in domain name
- extension (.com, .edu. .tw, .spam)
- hyphens or other nonstandard characters
- registration date
- registration duration
- WHOIS data (physical address, anonymous?, changes?)
- IP addresses referenced
Site-level
- 'discovery date' when Google first saw the site
- number of pages in the site
- number of inbound links to the site
- directory structre
- IP addresses
Page-level
- size of page
- number of scripts, images, css, other external linked/embedded objects
- title
- meta tags
- specific HTML tags (h1, h2, etc.)
- content
- keyword density
- keyword diversity
- link density
- number of inbound links
- anchor text of inbound links
- source of inbound links
- all those link elements for outbound links
- location within site
- inbound links from within the domain (relative prominence within the domain)
- update frequency
- update size
- discovery date
User Activity
- clickthrough rates
- estimated time on page
- bookmarking, social networking data
Competetive Data
- basically all these same factors for other sites/pages targeting same keywords
- update frequency, link popularity, and user activity especially
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That's all for now. Got any more? I'll add 'em to the list.