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.
Any site that take more than 10-15 seconds to start loading and more than 30 seconds to be at least somewhat visible, will be exited before complete loading in a large percentage of viewings. There are analysis programs that will tell you the amount of average time someone views your site. You will be surprised how short it is. No links to or from other associated sites. Links to and from sites that can be considered associated, are often overlooked as traffic sources.