Wow. New G5s (drool), and Panther was so so in my book, but I will love Exposé, the feature that elegantly lets you navigate windows. That rocked!
Fast search looked really cool too.
Fast user switch was awesome, even tho I don’t use it much. However, one might be inclined to now, simply for things like work vs. home uses on the same machine. You can almost use fast switching like virtual desktops!
And they rotate like a cube, too!