Google's 200 ranking factors

There's a great roundup of the factors, and how important they are thought to be, at SEOMoz

http://www.seomoz.org/article/search-ranking-factors#ranking-factors

On-Page (Keyword-Specific) Ranking Factors

• Keyword Use Anywhere in the Title Tag
• Keyword Use as the First Word(s) of the Title Tag
• Keyword Use in the Root Domain Name (e.g. keyword.com)
• Keyword Use Anywhere in the H1 Headline Tag
• Keyword Use in Internal Link Anchor Text on the Page
• Keyword Use in External Link Anchor Text on the Page
• Keyword Use as the First Word(s) in the H1 Tag
• Keyword Use in the First 50-100 Words in HTML on the Page
• Keyword Use in the Subdomain Name (e.g. keyword.seomoz.org)
• Keyword Use in the Page Name URL (e.g. seomoz.org/folder/keyword.html)
• Keyword Use in the Page Folder URL (e.g. seomoz.org/keyword/page.html)
• Keyword Use in other Headline Tags ( – )
• Keyword Use in Image Alt Text
• Keyword Use / Number of Repetitions in the HTML Text on the Page
• Keyword Use in Image Names Included on the Page (e.g. keyword.jpg)
• Keyword Use in or Tags
• Keyword Density Formula (# of Keyword Uses ÷ Total # of Terms on the Page)
• Keyword Use in List Items
• on the Page
• Keyword Use in the Page’s Query Parameters (e.g. seomoz.org/page.html?keyword)
• Keyword Use in or Tags
• Keyword Use in the Meta Description Tag
• Keyword Use in the Page’s File Extension (e.g. seomoz.org/page.keyword)
• Keyword Use in Comment Tags in the HTML
• Keyword Use in the Meta Keywords Tag

On-Page (Non-Keyword) Ranking Factors

• Existence of Substantive, Unique Content on the Page
• Recency (freshness) of Page Creation
• Use of Links on the Page that Point to Other URLs on this Domain
• Historical Content Changes (how often the page content has been updated)
• Use of External-Pointing Links on the Page
• Query Parameters in the URL vs. Static URL Format
• Ratio of Code to Text in HTML
• Existence of a Meta Description Tag
• HTML Validation to W3C Standards
• Use of Flash Elements (or other plug-in content)
• Use of Advertising on the Page
• Use of Google AdSense (specifically) on the Page

Page-Specific Link Popularity Ranking Factors

• Keyword-Focused Anchor Text from External Links
• External Link Popularity (quantity/quality of external links)
• Diversity of Link Sources (links from many unique root domains)
• Page-Specific TrustRank (whether the individual page has earned links from trusted sources)
• Iterative Algorithm-Based, Global Link Popularity (PageRank)
• Topic-Specificity/Focus of External Link Sources (whether external links to this page come from topically relevant pages/sites)
• Keyword-Focused Anchor Text from Internal Links
• Location in Information Architecture of the Site (where the page sits in relation to the site’s structural hierarchy)
• Internal Link Popularity (counting only links from other pages on the root domain)
• Quantity & Quality of Nofollowed Links to the Page
• Percent of Followed vs. Nofollowed Links that Point to the Page

Google's Eric Schmidt said that listing Google's 200 page ranking factors would reveal business secrets. Although Google does not officially publish their ranking algorithms, the guys at WebmasterWorld compiled a pretty comprehensive list.

Domain

• Age of Domain
• History of domain
• KWs in domain name
• Sub domain or root domain?
• TLD of Domain
• IP address of domain
• Location of IP address / Server

Architecture

• HTML structure
• Use of Headers tags
• URL path
• Use of external CSS / JS files

Content

• Keyword density of page
• Keyword in Title Tag
• Keyword in Meta Description (Not Meta Keywords)
• Keyword in KW in header tags (H1, H2 etc)
• Keyword in body text
• Freshness of Content

Per Inbound Link

• Quality of website linking in
• Quality of web page linking in
• Age of website
• Age of web page
• Relevancy of page’s content
• Location of link (Footer, Navigation, Body text)
• Anchor text if link
• Title attribute of link
• Alt tag of images linking
• Country specific TLD domain
• Authority TLD (.edu, .gov)
• Location of server
• Authority Link (CNN, BBC, etc)

Cluster of Links - Uniqueness of Class C address.

Internal Cross Linking

• No of internal links to page
• Location of link on page
• Anchor text of FIRST text link (Bruce Clay’s point at PubCon)

Penalties

• Over Optimisation
• Purchasing Links
• Selling Links
• Comment Spamming
• Cloaking
• Hidden Text
• Duplicate Content
• Keyword stuffing
• Manual penalties
• Sandbox effect (Probably the same as age of domain)

Miscellaneous

• JavaScript Links
• No Follow Links

Pending

• Performance / Load of a website
• Speed of JS

Misconceptions

• XML Sitemap (Aids the crawler but doesn’t help rankings)
• PageRank (General Indicator of page’s performance)