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So today I helped Dave out by converting his Drupal theme "Foliage" from 5.1 to 6.9. The theme was simple, and after a couple of hours learning what needed to be changed, the theme was up and working on my test site.
When Drupal evolves to a new version, it does not write code for backwards compatibility. This allows the Drupal core to be agile and nimble. Drupal upgrades are "read when they are ready", a significant difference from the scheduled Microsoft release and then patch releases. I believe the strategy is a sound one, allowing users to download solid code with high performance.
Drupal themes are very tiny, surprisingly so. Very efficient, small code, reliable core. It sounds like good software.