This answer is from Laura Melbourne, Director of User Experience at MIVA, Inc. She provided one of the longer answers.

The simple answer is not meeting the expectations of your users and/or potential customers. A users expectations are the threads that make up the fabric of the user experience. Knowing who your customers (current and prospective) is the key to meeting those controllable expectations. Through surveys, user-testing, personas, and ethnographic research we can get a good idea of who our customers is and what they want or expect. (I want it fast. I want it easy. I want to know what I am getting upfront. I don’t know what I want, but I know I want something.)

There are any number of factors that could stop a user from becoming a customer, (not finding what they are looking for, price, and indecisiveness come to mind), but your web site shouldn’t be the deterrent.

Ask yourself these questions:

• Is my site/app useful?
  (An essential user need is met… pay a bill, buy an item, etc…)
• Is it Usable?
  (Easy to use)
• Is it desirable?
  (Emotional needs are met through image, color, branding, etc…)
• Is it findable?
  (Can easily navigate through your site and find what they are looking for)
• Is it accessible?
  (All users are welcome, even those with disabilities)
• Is it credible?
  (Promotes trust and believability)
• Is it valuable?
  (Delivers value, improves customer satisfaction and/or advances the mission)

By utilizing the basic principles of usability and user experience strategy you will at least give yourself a fighting chance and offer your user a better experience.