I upgraded a local copy of MacMegasite to WordPress 2.7 beta 1, which took less than a minute and didn’t break anything. A major Drupal upgrade like that would result in hours or even days of pain trying to fix everything it broke.
I probably won’t upgrade the live site to WordPress 2.7 until at least the release candidate is available, but I’m sure it will go just as smoothly.
This illustrates the big difference between Drupal and WordPress. Drupal makes no attempt to preserve backward compatibility with themes & modules. They even consider it a virtue, making bigger advances possible. Unfortunately this causes lots of pain for people running Drupal-based sites and makes it very difficult to update.
WordPress, on the other hand, manages to maintain full compatibility between releases, yet they still manage to make a big advance in capabilities.