First, you want to make sure there isn't too much "action" happening on the screen at one time. There's nothing wrong with a scrolling banner, or a flash animation, but having more than one of these elements going at once is just too much.
Next, is color which can often be overwhelming. There are a lot of online color scheme tutorials and creators that can help you choose great colors. Usually the problems occur simply because the designer doesn't really understand the concept of contrast: bright on dark and dark on bright. A great site to help you to understand how color can work for your purpose is http://poynterextra.org/cp/index.html.Here are a few examples I've used as what NOT to do when creating a web presence:
World's Worst Website
Haven Works
These are some great sites for color design:
Color Scheme Generator
4096 Color Wheel