Examining the Role of De Facto Standards on the Web
Boxes and Arrows have published an informative article examining how common design elements on popular e-commerce sites have slowly become something of an unwritten ‘de facto’ standard.
This is something Steve Krug touches upon in his book "Don't Make Me Think!" - at the most basic level, people expect the top left of a page (or at least a corporate logo) to link them back to the homepage of a site.
The article contains a few more examples, but you get the idea.