An excellent article about visual metaphors in web design:
"Metaphors visually enhance the subject at hand. Metaphors make you think, inviting you to analyze how two subjects relate. On the web, visual metaphors can enhance content and a site’s purpose dramatically. This works magnificently if you seek to capture the attention of an audience for more than a quick glance."
http://www.devlounge.net/articles/visual-metaphors-7-rockstar-examples-on-the-web