eConsultancy have got a great article on the top 25 reasons why a user will 'bounce' straight away from your site. They list a wide variety of factors, some obvious like popups and slow load times, and other less obvious like using a template design that doesn't fit your content properly. It's amazing how many of them there are; and amazing how many websites break these basic rules.
If I was to pick the ones that annoyed me the most, they'd be: Read More...