"If you make customers unhappy in the physical world, they might each tell 6 friends. If you make customers unhappy on the Internet, they can each tell 6,000 friends. If you do build a great experience, customers tell each other about that. Word of mouth is very powerful." said Jeff Bezos, the CEO of the Amazon.com.
The list is split into 4 roughly equal sections, (I) Accessibility, (II) Identity, (III) Navigation, and (IV) Content.
This section contains not only traditional accessibility issues, but anything that might keep a visitor from being able to access the information on a website. If no one can load your site, or the type is too small to read, all of the usability in the world won't matter.
Page 2 of 2