Specifically, core needs support for HTML5 Form API so that contrib project maintainers can leverage the power of HTML5 forms. The markup for menus, scripts, styles, and published date is just too hard to change for people who aren't Drupal theming experts. They will get frustrated trying to make Drupal output HTML5. This is a good time to work on the default theme markup for Drupal core — the node, page, block, comment tpls, etc., as well as for Bartik. Core leads the community. Let's lead everyone into the future.
Presented March 9, 2011 at DrupalCon Chicago as an official core conversation.