Drupal 7

One of the things that a default Drupal installation does is that it handles 404s for static files within Drupal itself. In other words, a 404 for a .css or .jpg file causes a full Drupal bootstrap. This is not necessary, and wastes resources on a site that gets lots of 404s: lots of code is executed, many database queries are performed, only to return a 404 for a static file.

Debug function used for outputting debug information.

More: http://engineeredweb.com/blog/10/6/debugging-drupal-7

Drupal 7 and Drupal community features of importance.

it's getting better and more fun and more interesting with every passing moment. And that's because of the stellar contributions of the dozens of core developers on Drupal 7 (and 6 and 5 and 4.7 and 4.6 and 4.5), the leadership of people and companies in the Drupal community, and all the people who contribute back %u2014 whether it's out of the notion of "giving back" or out of the self-interest of inviting the rest of the community to help them on their project.

This revolutionary new release sports literally hundreds of improvements, and Angela Byron, the Drupal 7 co-maintainer, will lead you through the most important ones and how they'll impact your future site building endeavors.

See the associated notes of this excellent notetaker, in summary:
http://oitdesign.ncsu.edu/2010/04/20/drupalcon-2010-day-one/

This project will provide upgrade paths for schema changes in Drupal 7 until the HEAD-HEAD upgrade path is officially supported.