Good usability generally is unobtrusive and often goes unrecognised. By contrast, poor usability creates frustration and is disruptive to the customer.
Usability has always been an interest of mine. While I am interested in style, and it certainly also has a role to play in the user experience, an interface must be usable. It doesn't matter how good something looks, if it is difficult to use then it is worthless.
Accessibility is a key aspect of usability. Beyond that it is a matter of understanding the user, listening to their feedback and ensuring the system allows them to accomplish their task in the easist possible way.