On average, when you ask someone to perform a task on a site, they cannot do it. It's not their fault; it's the designer's fault.
Jakob NielsenMinimize the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.
Jakob NielsenEven the best designers produce successful products only if their designs solve the right problems. A wonderful interface to the wrong features will fail.
Jakob NielsenOn the Web, usability is a necessary condition for survival. If a website is difficult to use, people leave. If the homepage fails to clearly state what a company offers and what users can do on the site, people leave. If users get lost on a website, they leave. If a website's information is hard to read or doesn't answer users' key questions, they leave. Note a pattern here?
Jakob Nielsen