Happy Users are the brass ring of web site design. Ultimately, the designers of every site have making users happy as their primary objective. However, many designs end up frustrating instead. Since 1996, User Interface Engineering's main research objective is to understand how to achieve designs which produce happy users.
UIE's researchers have found that user happiness is directly tied to whether the users accomplish their goals on the site. What prevents users from accomplishing their goals? Almost always, it's poor design caused by a design team that doesn't have all the information they need to make the right decisions.