In case you missed it, OStatic's Sam Dean has an excellent interview with Angela Byron about Drupal. Angela works for Lullabot and is an important contributor in the Drupal community. She contributed to the upcoming O'Reilly book Using Drupal
The Drupal project has an unconventional philosophy on backwards compatibility. During each major version of Drupal, developers are highly encouraged to think up crazy new things that Drupal can do, without fear of breaking legacy APIs. While users' data will always be preserved throughout the ages, if we come up with new standards we want to support, or a much better and more performant way of doing something, developers are given free reign to go off in that direction.