aristotle - 4:15 pm on Sep 29, 2012 (gmt 0)
There is also the issue of Matt Cutts saying Google's algorithm is meant to identify "quality" sites. They think they can do this with a mathematical algorithm which, when you think about it, is really impossible. It takes a human with a knowledge of the site topics to evaluate (by actually visiting and browsing the site) whether a site is a "quality" site, and even there subjectivity will enter the picture.
Their original main signal of quality was backlinks. If someone links to your site, at one time that was normally a signal or vote of approval. But because spammers (or linkbuilders) began creating "unnatural" backlinks to game the system, that signal has lost much of its value.
So now Google may have started using other signals of quality in addition to backlinks. For example, if visitors tend to spend a lot of time exploring your site, bookmarking pages, printing out pages, returning for repeat visits, etc, then these are other possible signals of quality that the algorithm can use.