How come the default theme Twenty Eleven fails validation test, both for HTML5 and CSS3? I checked: http://twentyelevendemo.wordpress.com/
Should I be bothered that the validation fails?
Validation is a tricky topic. It is good to have, but it is also theoretical thing in practical world.
My opinion is that unless you are dead set to only using perfectly validating themes on principle, then Twenty Eleven is not for you or you will need to create child theme and clean it up.
If you are concerned that Twenty Eleven just might have horrible markup/CSS issues - that is unlikely in theme of such high profile.