The W3C have a super neat online demonstration of visually representing web accessibility. The inaccessible pages have several “barriers”, key elements that make the page inhospitable. For example, the before home page demonstrates a lack of alternative text for each image, an inconsistency in the content order, a negligence with headings and lists, an inaccuracy with link texts and font sizes.
Even by visually comparing both inaccessible and accessible pages you can see the subtle, yet important, differences. I suggest to examine one page at a time. Carefully look at the before and after of the home page, then read the “accessible barriers”. When you understand the “barriers” then go on to the next page.
If you want to see how to go from a non-compliant site to an accessible one, you need to read this document, the before and after demo at: http://www.w3.org/WAI/EO/2005/Demo/Overview