Don't Surprise Me
When I visit your site I want information. Your content might surprise me, but your navigation should not.
When we use books and newspapers we rely on layout conventions. We know that the index of a textbook is at the back. We find our way through a dictionary using the words in bold face at the top of the page. The most important stories in the newspaper are on the front page.
The web has a similar set of conventions. Some of the more important ones are "the name of the site should be displayed at the top of every page", "menus are at the top or left", and "links should be underlined."
There are times when some of these rules can be broken, but care must be taken if you don't want to loose readers. If they have to think about your navigation, they're not thinking about your content.