Simplicity
While your web site should be dynamic and interesting to your audience, it is most important that it is easy to use. People will come to your web site for a reason and they'll want to be able to access the information they want. Remember this acronym in providing a successful web site :
H - High-quality content
O - Often updated
M - Minimal download time
E - Ease of use
R - Relevant to users' needs
U - Unique to online medium
N - Never stop analyzing your audience, evaluating your site, and evolving