It is not only knowledge of the accessibility guidelines that is important. One must understand them as well, and understand what are the underlying issues. If we go about implementing each guideline without understanding why we’re doing what we do, we risk ending up with pages that are meeting the guidelines, but that are not accessible!
For example, adding a summary to a table used solely for layout is pointless. It gets in the way of reading the page when using a screenreader, and provides absolutely no valuable information. While we know that nested tables are a Bad Idea, they still happen. And if we add a summary="This table is used for layout" to each table, we quickly end up with a page cluttered with irritating information. Yet, it somewhat respects WCAG!
Similarly, the alt attribute of the img tag. We have to consider what the image is. Are you using spacer gifs? If so, don’t add alt="spacer image", because for each instance of your space, the words "spacer image" will be announced. How quickly do you think that will chase people away from the site? The topic of alternate text for images is a complex one and is discussed
These are just two examples of why it is important to understand why we do these things. Otherwise we risk applying guidelines willy-nilly, and potentially causing more problems than if we hadn’t bothered in the first place.