And it's not just the characters - it's the show itself. And that's something I really respect about the show's writers and producers. I've put my thoughts about that here (http://www.livejournal.com/users/imation23/973.html), ages ago. I think that's one of the reasons why AtS is still going strong in its 4th season, while BtVS started to become stale at around that time.