Often we forget to make things easy for those who aren't web savvy. Small things can add up to big confusion if ignored. Do this for all fields where the input could vary, such as date formatting. If you're asking for information that seems incongruous or requires some specific knowledge, provide a link to clearly define the question. Every chance to confuse a user is a chance you may lose the user.
User testing can improve site usability and functionality.
One web site had a problem with its login screen. So when users got errors, they became confused and frustrated. Watching several users make the same mistake allowed the company to make a small fix and greatly improve the ease of registration. Now new users see the button first, before they type in a username and password. User testing helps you identify potential high-risk situations that create consumer confusion.