HTML5 still has alt="" and still has headers=""; indeed, it defines them in much more detail than HTML4.

Ian, you're right, I wasn't specific enough. Regarding alt, I said "which drops support for several accessibility features, including the alt attribute for images" when I should have said "makes it possible for alt="" to be omitted." The comments on Gez's post flesh this out.[1] On headers, the issue is even more curly: they are in the spec but do not apply to the td element. Gez's bug is still open on this,[2] so the accessibility issue remains a question. I've never made a table remotely like this,[3] let alone seen one marked up correctly, and I didn't appreciate the complexity of the issues James raised in his post until you called them out, but from what I can tell these two accessibility issues are still unresolved for HTML5.

I think it’s going to be much easier to get computers to understand humans than it would be to get humans to understand how to write computer-understandable content like RDF, let alone motivating most humans to actually write such content.

I think I agree with you. The argument that it's easier to change computer behaviour than human behaviour seems pretty sound.

[1] http://juicystudio.com/article/html5-alt-text-authoring-tools.php
[2] http://www.w3.org/Bugs/Public/show_bug.cgi?id=5822
[3] http://juicystudio.com/wcag/tables/complexdatatable.html