Does Google consider anything other thank just links when calculating your website's PageRank, e.g. the number of google users who visit your website, the amount of content in your site, etc.? Or is it just number and quality of backlinks?
Amount of content and the actual content of your content is taken into consideration. The number of users visiting your site may also have an influence. Google do collect some information through the Google toolbar.
Just backlinks, content or visitors are not used for PageRank, the only time content will play a role in pagerank is because of links on the content page.
Complete and utter rubbish - Google's own definition of Pagerank PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page's value.
Backlinks plus outbounds, and probably other various damping factors. The PR algo has changed a lot over the years, but it's all speculation, since the horse ain't talking.