There are just as many ways to frustrate and annoy visitors as there are ways to attract and satisfy them. Try to avoid the things that make visitors hit the back button and never return. There are some things that are borderline and won't necessarily annoy the visitor, but may make them ignore content.
Don't use popups or popunders
Unless it is clear that clicking the link will open a new window, and never set them to automatically open. Visitors find them distracting and annoying, often ignore what was in the window and associate them with advertising.
Don't make your pages so large that people have to wait for them to load
Many people are still using dialup, don't make them suffer through endless page load times. Try to keep file sizes down, use CSS to keep overall page load times low and don't overload pages with heavy graphics. Remember to optimise graphics so the files sizes aren't larger than they need to be.
Don't design so that your site will only display in one browser/platform or screen size
Remember to make your layout resizable so it will work on different sized screens. Check that it displays correctly using both PC and Mac computers, using Internet Explorer, Firefox and Safari.
Don't use animations for the sake of it
Animations should be used sparingly. They can attract attention when first seen, but are also distracting and make the other content hard to focus on.
Don't use colour for the sake of it
A good colour scheme is fine, but a combination of intense colours such as bright pinks, yellows or greens as text or background is difficult for visitors to read and concentrate on.
Warn about PDF or other download formats
When adding resources such as PDFs, make sure visitors are aware that it is a download, not another webpage that they are clicking on.
Don't use FLASH alone
If you really want to use FLASH for an intro page, make sure there is an easy to find option to skip the intro. Many visitors just won't wait through a FLASH intro, they'll leave the site. For users on dialup, an option to skip the download can be the difference between entering a site immediately or five minutes later, which does nothing for their first impression of a site.
Avoid making content look like advertising
Advertising is so prevalent online that visitors now "tune out" anything that looks like an ad. Banners, skyscraper images on the side or anything resembling the image sizes common to advertising are literally not seen. Likewise layout that mimics ad formats such as adsense are often screened out as well. If your content or navigation looks like this, chances are visitors may not even see it.