How does Google figure out relevance in realtime? With twitter, it's user driver content with tags etc.
But with the net at large, blogs etc, this becomes difficult. Incoming links etc are hard to determine in real time (primarily because they haven't occurred yet).
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.
But with the net at large, blogs etc, this becomes difficult. Incoming links etc are hard to determine in real time (primarily because they haven't occurred yet).