For those who haven't really been following it, several hundred contributors, 13 months, and tens of thousands of lines of code have gone into making the only version of Drupal ever that is better than Drupal 5. So, naturally, we've released it and called it Drupal 6.
Drupal 6 boasts a boat load of new functionality, ranging from Ajaxy yumminess throughout the system to native support for OpenID to vastly enhanced multi-lingual support. Several entire subsystems have been either overhauled or totally rewritten to proivde more power, flexibility, and speed. The official press release has the complete rundown, or for the more visually inclined there's a new features screencast. For me, though, the new theming system is feature numero uno.
At Palantir, we specialize in making sites that don't look at all like Drupal. That means we're frequently bending the theme system to its limits and doing some wacky and crazy (but cool!) things with it. Often, though, we find ourselves running into places where something is more complex than it needs to be. In Drupal 6, the theme system has been almost entirely rewritten to be more template-centric. That's great news for our themers, as it means there's more they can do without having to bother the programmers (me). That makes it great news for me, too. :-)
Moreover, important changes in the way Drupal's default HTML is built make skinning it much easier. That, in turn, means CSS wizards can do far more with just CSS, without having to mess around with template function or, god-forbid, PHP. Even if most high-end themes will still involve some HTML and PHP work, the more design work can be off-loaded on CSS, the cleaner the whole theme becomes. It's even possible for modules to inject new values into each other's template files cleanly, for vastly improved system modularity.