How do search engines decide site rankings?

I may have written about this or mentioned it: folk finding you via search engines.  How do search engines decide which websites to place higher in their listings than others?

That’s a well kept secret that lots of folk get paid lots to try to second guess. It’s the field of Search Engine Optimization (SEO). This much is known: search engines value more the sites they think are more important.  So let’s start working from there.

What’s their idea of important? Well here are some things they likely consider in no particular order:
* number of visitors
* length of time visitors stay on a site
* number of other sites that link to the site
* how fresh the content is
* how helpful the content is
* how much helpful content there is
* how clearly organized the site is – how easy it is to find what the visitor needs to find on the site
* does the site provide what it says it’s going to provide?
* how unique is it for its sector or geography or?

What other things would make your site important in the search engine’s eyes?

What else would make it important to your visitors?