Americans with Disabilities Act (ADA) Compliance is now a scary term for high-volume eRetailers. It hasn't been decided in court, but Target has been sued in a class-action lawsuit for not making accomodations for disabled visitors.
The language of the Act is pretty vague and general, and those with bigger budgets are expected to make correspondingly more accomodations, and to muddle the muddy waters further, there's just no list to go by to say what is compliant and what is not.
There ARE however a couple of things that are just good practice anyway, the first of which using alt text for ALL images (except maybe corners on tabs and things like that). Sight-impaired visitors have devices that read the alt-text out loud so they can know what's on the screen in front of them. It's also a best practice for Search Engine Optimization, so just do it!
Another requirement from a recent client was to make the site keyboard navigable. This was a doozy. We had a pre-packaged component that came with aspdotnetstorefront, but it was a no-go for keyboard navigation. Our senior developer on the project, Tomas Vera found this nifty menu at http://www.milonic.com that you can tab into and use your arrow keys to navigate. This is a great find, IMO. Another tip for you, set your tabindex to 1 on your website logo so that if a user wants to tab through the page, they will always start at the top left. The only exception to this would be a search page where the user expects the focus to be in the search box so they can just begin typing.
Just one more disjointed rant from the trenches!