Section 508 vs. W3.org/WAI Guidelines
Essentially, the rules in the United States Section 508 legislation represent a small subset of the guidelines put forward by the World Wide Web Consortium's (W3C) Web Accessibility Initiative (WAI). Section 508 standards are aimed addressing a minimum set of barriers to information access that should be avoided by any website. The WAI's Web Content Accessibility Guidelines are much more comprehensive. The W3C's list of priorities goes further to ensure that websites are actually usable by the broadest possible ranges of people.
At Jelve Design, we believe that it is possible to conform to the Section 508 legislation, yet still have a site that is very difficult for many people to use. If your primary interest is to limit potential liability at a minimum cost, then simply following the Section 508 rules may be your best option. If it is important to for you to make your site easily usable and to present your information in a meaningful way to the disabled community, however, then you may wish to consider going further and following more of the WAI's guidelines.