1. Animation Overload: Using animations excessively on web sites would put your visitors off. Beware of blinking text and images. Only use animations where it would add a value to your content.
2. Color combinations: Don't just use the colors because they are your favourites. Choose right color combinations for your text and background. For instance blue over red background makes it difficult to read. Go for the colors that are easy on the eyes. Choose web safe colors.
3. Lack of Maintenance: Make sure all your links are working properly. Also, make sure your site loads correctly. Here is a free tool to check dead links on your site.
4. Built for one Browser: You have to keep this in mind that there are other browsers as well as Internet Explorer and they are also used wisely. Your site may not render as the same way it appears in your own browser. So make sure to test your site on different browsers and different machines.
5. Using Tables Excessively: Tables may not render same way in the old browsers. And some browser may not support tables. So try using table wisely.
6. Using Images as Bacground: When you using images as your background, make sure it doesn't distarct your visitors to read your content.
7. Pop-up, Pop-unders: Don't give your visitors something they don't expect. In most cases, users will press the back button or close button of the browser as the pop-up and pop-unders appear. Try to avoid using them.
8. Excessive Advertisement: Always label your advertisements as sponsored links, advertisement, or anything similar. Don't expect your visitors to find your main content between advertisement. Because they won't. Dedicate a space in your site for advertisement and label it clearly."