I haven't had much time to play around with the themes other than the included default themes, but from what I have seen so far, the new themes are very slick! Most noticable is that you can now choose colors of different components with an Ajax / kde style color picker.
I created something similar for myclassroomhelper, where the entire theme was created using png transparency. The creaphics created a two tone style, and the user could set the color of all the elements which were stored in a mysql backend and used to generate dynamic stylesheets. All of the drop shadows and bevels were created by the foreground png graphics.

The default drupal theme seems to be doing pretty much the same thing, but the Ajax color picker is much more elegant than my table javascript solution. I am impressed, and can't wait to create my own Drupal 5.0 themes.