If your client asks you to support WCAG level AA accessibility guidelines, read on to learn how your project will be impacted for designers and developers.
(1) There are contrast ratio rules for text on a background that must be followed for AA that do not exist for A. I use this contrast checker, which provides pass/fail criteria based on font size (http://webaim.org/resources/contrastchecker/). Make sure your designer is aware of these rules.
(2) Time based media impacts: "If A level means that you provide captions for pre-recorded audio content in synchronized media (unless there’s a clearly labeled media alternative for text), then AA means that you provide captions for all live audio content in synchronized media and that there’s an audio description available for all prerecorded video content in synchronized media.” (Taken from this article: http://blog.echidna.ca/article/aoda-and-you-alphabet-soup-wcag-20-or-aa)
(3) You have to be able to double the text size through the browser and have the text still be legible for AA. This requires an additional unit test for front end developers and for the QA team. Sometimes it is not apparent how the text should appear as it grows and needs design input. Test this out in Firefox with View > Zoom > Zoom text only.
(4) This next one is applicable if the client is relying on text embedded in images for any of their campaign banners, for example: "If the same visual presentation can be made using text alone, an image is not used to present that text.” (See more info here: Web Content Accessibility Guidelines (WCAG) 2.0 -- 1.4.5).
(5) A visible focus indicator is needed for AA. (As you tab through the site, you can see which element has focus). This benefits from a designer’s input and should have client sign-off on the look and feel of the focus indicator. (See more info here: Web Content Accessibility Guidelines (WCAG) 2.0 -- 2.4.7).
(6) Different languages within a page need to be marked up as such with a lang attribute.
(7) Rules on input errors for forms ("If an input error is detected (via client-side or server-side validation), provide suggestions for fixing the input in a timely and accessible manner.”) This one is a best practice anyway, but is simply called out as a requirement for AA whereas it is not for A.
*NOTE: This list doesn't include all of the level A criteria, just the additional criteria required for AA over and above A.