Unclear focus. If the central message(s) is difficult to find, visitors will have to work too hard. Web visitors don't read, they scan. Website information needs to be clear, concise and complete or the visitor will lose interest and move on.
Annoying design. Ugly, glaring color, instant sound, or too much motion drives visitors away.
Too much text and unorganized content. Reading online is harder on the eyes than reading a book or newspaper. If it's difficult to find information, the visitor will leave and go somewhere else.
Bad content. Meaningless content, out-of-date content, or too much marketing/technical language quickly drives visitors away.
Dominated by advertising. Study after study shows that web visitors generally avoid anything that looks like a blatant advertisement. Web visitors are looking for information, not ads.