The site's PR, uptime/age, update rate, uniqueness of content. By this measure HN is near the ideal: it has amazing inbound PR but does not link out much. It's been running fast and fine for ~3 years, the content is often unique, in the sense that it contains phrases Googlebot has never encountered before.
An experiment: here is a search that matches an exact phrase in this comment. http://www.google.com/search?q=%22By+this+measure+HN+is+near...
At the moment it returns nothing. within a minute or two this comment till be the first result.