History of Feminism

Buffy had strength AND vulnerability. She was also able to express emotions like fear, sadness, etc that typical 'badass' male heros are never allowed - which in the end (even thought she was partly an 'action' character) makes for a more human and powerful character.

I think they did a decent job showing that Buffy's ability to work with the other characters (who were often considered 'weak') was what allowed her to 'win'.

In addition to of course actually writing a decent number of female characters and character development in the series, who didn't rely on male support.