Less obvious difficulties, and ones that sometimes get forgotten:
OK, I've made my site accessible for people using JAWS, and have got that awkward guy next door who won't use a mouse to test it too - can I go home now?
Possibly, but only if we don't want to include people accessing the Web through mobile devices, geeky people like Smiffy who, for strage reasons, access the Web over an SSH connection and Lynx, the larger than one might think number of people with some degree of dyslexia, those with learning difficulties...
We haven't even scratched the surface. Just reading WCAG10, we will find that there are an awful lot more difficulties that we need to address, and these aren't neccessarily any kind of minority either.
So, diligently, we review what we have done, make further changes, performed user testing with the "usual suspects" and are fairly certain that we've acheived AAA compliance and can now happily knock off and go down the pub. Woohoo!
Three years ago, I might have thought this. Personal experience, however, has now added a whole new dimension to Web accessibility, and it's one that doesn't get mentioned an awful lot.
To quote Rolf Harris, can you guess what it's going to be yet?