The Anatomy of A Perfect Website Infographic has some useful data on the staples that make a website excel. This information includes Web Design, Navigation, Usability, Content, SEO, Social Media, Tracking and Analytics, and tips on the Footer. This infographic is from November 2011 so it does not take Pinterest into account in the Social Media section. From the chart, the most important elements of a website seem to be usability and content. I believe that content edges out usability a little. Look at the infographic below and decide for yourself.
The Anatomy of A Perfect Website Infographic:
The Anatomy of A Perfect Website Infographic