Most Important Web Usability Issues

Usability is crucial to create Web sites that your customers will want to return to. If they can't use the site, they won't stay. While there are many specifics, there are some simple steps you can take to create a site that is accessible and useful for your readers.
1) Content
2) Page Layout
3) Colors
4) HTML
5) Download Speed
Most Important Web Usability Issues - Read in details (Courtesy: About.com)