Functionality is to me the single most significant factor in web design. A site should work exactly as it is meant to, and it should be easy to navigate, even for someone fairly unfamiliar with the web.

Your navigation should be intuitive. You should be able to get back to the main page, and preferably navigate around within the site, from any page.

If the site is large or a little more complicated - say with different levels of pages within the site - then it may be a good idea to include a site map, which can be accessed from anywhere within the site if the user becomes disorientated.

Assume nothing, and test it out on everyone you know, in as wide a variety of operating systems, platforms and browsers as possible. It may not be as easy as you think.