With regard to this thread, I am very supportive of the general discusion. Do want to make some distinctions.
Validity compliance - here we're talking about XHTML and w3 specs and doctype, etc.
IMO, that is a basic starting point.
Beyond that, there is accessibility. Here we need to avoid things like menu systems which rely on javascripted postbacks, as they are not screen-reader friendly. Additionally, over-use of tables for non-tabular data can provide difficulties. Other basics such as alt tags for images etc I expect all are familiar with.
Finally, usability itself is a third factor which would use the first two as building blocks but goes beyond them in terms of simply-navigated sites. Here, design, accessibility and validity all meet in support of the result.
It is easy to mush these issues together but in reality though they do affect each other they must be tackled sometimes in different venues.
Looking forward to the fruits of Cathal's labors and others as well. Did I read somewhere that SolPart is developing a new menu system that is screen-reader friendly?