To determine the most important aspect of a site's functionality requirement, ask the question "what is this site's purpose?" The purpose describes the function the site must fulfill. In the world of the internet, functionality is synonymous with good linking of your internal pages. So, there are really two distinct areas of concern when it comes to functionality, The first is quality internal and external linking. The second area of importance regarding functionality is how well a website performs it's purpose. If the purpose of a website is to deliver a message or convey a concept, is it doing this in a logical and interesting manner that holds the attention of it's intended audience?
If the website is selling a product is it doing it in a way that is convenient and easy to understand?