Testability - The decision to comply with the testability requirement has outlawed some very important success criteria. This requirement has also not been applied fairly to all success criteria. Some current success criteria do not comply with the 8 out of 10 rule - for example 1.1.1. This SC requires that alternatives to non-text content "serve the same purpose and present the same information". One basic example of this is to require equivalent ALT attributes for images, but I do not believe that 8 out of 10 people would agree on the same ALT attribute for an image. For instance, in the Live in Victoria site (www.liveinvictoria.vic.gov.au) there is an image under the heading "Business Migrants". When I worked on this site, several people said this image should have a null ALT attribute as it conveyed no information. Several other people suggested ALT attributes of "A couple of business migrants chatting at work" or "Guys chatting at work". Whereas the ALT attribute that I recommended was "There is a wealth of opportunities for Business Migrants in Victoria."
Remove the requirement for testability and set up a taskforce (I volunteer to work on or head up this taskforce) to identify criteria that should be included in WCAG2. Alternatively, develop a set of supplementary guidelines that are non-testable to be used in conjunction with WCAG2.
The success criteria need to be testable or else people cannot tell when they have conformed to the success criteria and thus WCAG 2.0.
With regard to SC 1.1.1 the success criterion does not require that ALT text provided by different people be the same. WCAG 2.0 categorizes different classes of alt text and provides test procedures to help humans evaluate whether alt text satisfies the success criterion.
Advisory techniques are used to provide supplemental materials that are non-testable that can be used in conjunction with WCAG 2.0. They provide additional guidance on what can be done beyond the requirements of WCAG.