Inadequate use of usability engineering methods in software development projects have been estimated to cost the US economy about $30 billion per year in lost productivity.

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.

...pay attention to what users do, not what they say.

Progressive disclosure defers advanced or rarely used features to a secondary screen, making applications easier to learn and less error-prone.

People have to want to change before there's any chance of helping them do so.

A general principle for all user interface design is to go through all of your design elements and remove them one at a time.

Consistency is one of the most powerful usability principles: when things always behave the same, users don't have to worry about what will happen. Instead, they know what will happen based on earlier experience.