Let me start by saying layout and design are important. For the casual observer, usability means a visual design that is pleasing to the eye. Historically, web designers came primarily from print design backgrounds and as such, focused their talents on the static visual design aspects of early websites; headers, icons, logos, backgrounds, etc. While important, the implied focus of web design is too heavily weighted on expressing the individual creativity of the designer than it is on satisfying the needs of the customer.

Commercial websites and application must primarily address end user interactions to complete tasks efficiently and effectively. To that end, consider the following non-visual aspects of usability:

1. Easy to read url's
For B2C websites, maximizing customer recall gets them to products sooner so www.yourstore.com/electronics/ is preferred over www.yourstore.com?cid=12. For any site where users offline starting point is a mass mailing, or call center gateway customer service telephone call (government agencies/ product companies/ political campaign direct marketing) the intended audience has to be able to read, remember, and type the url to affect the intended conversion.

2. Hire a good technical writer
We recently completed an assesment for a VOIP startup company where it became clear early on that a "Quick start" guide would ship with the hardware component of the service. Presenting business concepts succinctly, in terms the end user is already familiar with, lessens training time and improves user acceptance.

3. Focus on system messaging
Error recovery is more important than error prevention. Meaningful error messages allow users to build confidence when learning complex systems. No system will be successful if the end users fear using it.

4. Task flow consistency
Similiar tasks should happen in similiar ways. Beyond consistency in the layout of tabular data, managing the data sets should happen consistenly across the system. Avoid doing things one way in one part of the site and a different way somewhere else.

5. Import/Export File formats
Can your users export data from your system , edit it, and import it back in? If so, make sure these files are  as human readable as possible.