One thing that attracted me to WP was XHTML standards. The developers and community have worked hard to keep this going. I'd like to see the default template come with valid CSS and a link to a CSS validator as well.
I don't profess to be a CSS expert, nor would I be willing to be money that my own site is valid CSS, either. It just seems to me that if the code is going to be valid, then we should go the extra bit to make the CSS valid, too.
This can be an enormous challenge, of course, given the broad spectrum of browsers in use out there. It's not an impossible thing, either. There are some important basic rules in CSS that are often not followed, and we all should strive to make our sites as close to valid in both areas as possible.
Comments? Suggestions? Go soak your head, Nuclear Moose? :)
I'd be willing to take all of the default CSS in the WP install and ensure that it is validated.