You can design for emotional instability actually, I've found from working with people suffering severe emotional and psychological disabilities from everything from dyslexia and anger management to schizophrenia that making your website work seamlessly (no errors), ensuring your design emotes the right kind of feel (no aggressive flashing images or intrusive components), the use of soft colors, good contrasts, and using "neutral language" (IE: None aggressive or trollish comments in content) will often reduce the anxiety and stress which cause people suffering emotional problems to suffer. Just thought I would add to this as I didn't want people to think it's not possible to design with these people in mind (I recommend people buying a book on psychology to gain a better insight into their users).
One of the main problems with disabilities is that people tend to reduce them to a childlike state. The point of accessibility is to make things work and function for a wide range of people with impairments, not to sacrifice everything to mollycoddle them and treat the end user like a baby. The disabled hate being talked down too, and they won't appreciate stereotypes, sweeping statements or false assumptions and dumbing down.
Precisely, I think "picking" favourites among the disabled community is both counter productive to the cause and it's rather insulting to people involved. Emotions aside I would prefer people to be pro-active with needs rather than perceptions and checklists of restrictive issues.
Sorry to tell you this Dave, but your totally wrong, Section 508 (which I'm assuming your hinting towards) does indeed apply just to government sites, but that doesn't mean inherently that there's no laws which speak in general about websites needing to be accessible. In the USA there's the ADA (American Disabilities Act), in the UK, Australia and Canada (and several places in Europe) there are variations of the DDA (Disability Discrimination Act), in Italy there's the Stanca Act, in Germany there's BITV... and there's others including a new EU act on the way (Stomme can post it, I can't remember the name, lol)... all of these implicitly apply to disability relating to public services (some even extent to personal websites). So yes... making a website inaccessible (especially a commercial one) is now a criminal offence.
Even more interesting, I know someone involved in accessibility law and he told me that because the UK's DDA is so wide reaching (due to international law and how the UK legislation applies outside it's borders), if an American website owner who hosted his website on US soil refused to amend his commercial website to make it meet something close to compliance, and a UK citizen suffered as a result. The UK citizen would be entirely within their legal rights (as a disabled individual) to follow up legal action against the person in the USA for violating International law (especially with the new EU laws coming in) and they could have the person (under an extreme case) extradited to this country, charged and convicted under UK law for criminal offences against UK citizens (in regards to committing an offence against anti-discrimination laws). Sounds pretty scary I know but I've read the laws and the factors involved seem pretty fair in that they'll only hit the worst offenders in serious circumstances (and perhaps we'll weed out some accessibility damaging "web professionals" too).
Stupidity may be an illness (of sorts) but there's only so much you can do about it