In the age of high speed Internet, everyone has a website. The most common mistake found in web design is that the designers give the page a subject, but not a focus or a purpose. When your website has a purpose, the design will reflect that in the use of images, supporting software and links. When it just covers a subject, the generalization can make the design clunky and overwhelming.
Purpose limits the content. It does not require a dumping of every single factoid related to the subject. Web design avoids the crutch of too much information distracting visitors from the purpose of the site. A business wants their user to return, not to leave in frustration because overwhelming data keeps them from finding what they need.
Without purpose, a web design cannot advertise well. It may confuse potential clients. The last thing your web design should do is leave clients confused or turn away a potential paying customer. A focused design will detail the business purpose, the services and clearly demonstrate the work either through samples or service descriptions. Web design is about the presentation of the purpose, not the subject.