Just a small addition to the list of possible accessibility problems, mostly of technical nature:

• Cookies disabled. So many online shops fail when user tries to place an order. Some are intelligent enough to notify about the problem. When shopping from security of my home, would I trust a merchant that has to stuff my computer with cookies at a cost of the order lost? Is that a secure place to buy from?
• JavaScript. This is to add usability. If the feature is turned off in the browser for security reasons, the user should still be able to accomplish all the tasks. The http://www.national.org.nz/ is a home of one of two major political parties in New Zealand. While on the site, go to security settings in your browser, turn scripting off, refresh the page and see parts of it disappear. Very "professional"! And these guys are trying to persuade the public that they are taking our security seriously! How do I access their site from my home where my browser is secured?
• Client side errors on. Many web developers have client side errors pop-up switched on. They are potential online customers more than anyone else. They also know that the secure sites do not have errors. Enable the client side errors notifications in your browser and try to use menu on the home page of the abovementioned website.
• Slow connection or server. A lot has been said about that, but there is one aspect usually not mentioned. You start reading a web page, and out of sudden the text starts moving giving the space to images that keep being downloaded. Usually these poorly designed pages have plenty of images to annoy the user. Please remember that the transition of the images from your server to user's browser takes time, so let the browser know beforehand how much space to reserve for each image.
• Pop-ups blocked. Nowadays the Pop-up blockers protect us from being assaulted while shopping from security of our homes. The pop-ups are blocked by Microsoft security roll-up, Google toolbar, etc. Still, there are many sites that rely on pop-ups to deliver core functionality. If I have to defend myself from your annoyances, do not expect me to buy from you?
• Small screen. While there was an ease in web development when user’s screens, even at home, were becoming larger and of higher resolution, nowadays we are back into age of small screens. The mobile devices currently can’t have big screens due to their very nature. Whatever your targeted audience is, it is best to ensure that the user does not have to scroll horizontally. If you are selling cheap stuff, cater for users with cheap monitors and dial-up!