'Usability' is refered mainly with respect to user interface of information system, for example, design of Web site. It is defined in ISO 9241 (http://www.iso.org/) as follows :
"Usability is a measure of the effectiveness, efficiency and satisfaction with which specified users can achieve specified goals in a particular environment."
The inquiry of usability takes course of (1) reducing personnel requirements and (2) reducing training requirements.
But, in fact, 'usabiltiy' concerns to all of information type, infomation design, user interface design and diversity of individuals.
The point is that human factors must be considered from both sides of common features and diversity.
For example, "hesitant users (contrasted to frequent users)" is a problem to be approached in this way. — Distinction between can't and won't becomes essential.