John Albin Wilkins (aka JohnAlbin) talks about some of the new extensions to the theming system that are new in Drupal 7 including render elements, hook_page_alter(), show, hide, theme_get_suggestions(), and improvements to theme_links(). He also talks about the theme administration page overhaul, how it's now a lot easier to create "mega-menus" dropdown panels when hovering over the primary navigation, as well as another presentation that Palantir gave on previewing Drupal 7.

For more detailed information on what's new in Drupal 7 for themers, then be sure to check out John's DrupalCon presentation titled "Design and Theming: What's new in Drupal 7?"