One of the most important responsibilities of a website is to communicate with its visitors. Regardless of whether the site is a blog, a portfolio site, a corporate site, or an e-commerce site, it exists to communicate with visitors.
The communication in some cases is two ways (blog comments are an example), but in many ways the website is communicating to visitors before they even offer their feedback. In order for the site to communicate effectively the designer and site owner need to clear purpose of what messages should be evident to visitors.
In my opinion, improving the communication is an ongoing process. As I evaluate my own sites, or those of clients, these are some of the factors that I consider.