You will be surprised at some of the names available. Too little information given. Giving too little information leaves the visitor confused. Be sure to clearly state the purpose of your site and provide the information that will allow viewers to decide if they want to either proceed deeper into your site or to contact you. Too much information presented.
Most sites provide too much information thereby confusing the viewer.
The other big mistake is long pages of text. Break up the information into several pages and reduce text to a minimum. What is easy to read on hard copy is often difficult to read on a screen. Too much graphic overhead. Graphics are a very important part of a sites presentation.