She showed a very good (and, for the genera, realistic) teenage girl progression. From being rich, popular, and alpha-pretty girl in High School, to being forced to find her way in the real world, she grew up. You see many of the others do no such thing (Willow goes Team Tara, and changes her clothes to match, Xander stays man-child ultimately running away from long-term commitment, and Buffy's only character progression comes after being forcibly pulled from Paradise to come back to Earth).
Hollywood has never been good at loosing.
This is another example.
~Jeordam
Saving the Princess, Humanity, or the World-Entire since 1985