Psychologist Christopher Ferguson has done the empirical research, put in the legwork and figured out that Buffy is awesome, which I absolutely could have told you. More specifically, his piece in the Journal of Communication chronicles the results of 150 Texas A&M students' screenings of three disparate types of television: those with sexual violence and strong female leads (Law & Order: SVU, Buffy the Vampire Slayer) those with sexual violence and subordinate female characters (The Tudors, Masters of Horror) and a neutral PG-rated test group (Gilmore Girls, 7th Heaven).
Afterwards, he asked the students to rank true or false questions such as: "A woman should not expect to go to exactly the same places or to have quite the same freedom of action of a man," and, "The intellectual leadership of a community should be largely in the hands of men."
The men who viewed The Tudors had more sexist responses to the questions overall, while the womens' questionnaires remained unchanged regardless of which group they viewed. Men had a low anxiety rate while watching sexually violent shows with weak women, but when they watched ladies kicking ass on Buffy and SVU, they registered high levels of anxiety—and answered the questions above with less sexist douchebaggery. The women, meanwhile, when watching The Tudors and Masters of Horror, unsurprisingly "registered higher levels of anxiety than were present in any other category."
While it's not mentioned in the journal, it's disclosed on on Slate's write-up that one notable factor here is that the students were:
150 largely Latino college men and women watching media overwhelmingly featuring lily-white women.
Now the duh part: Ferguson posits that watching strong female characters on television reinforces a positive image of women, while subservient female characters reinforce a negative, sexist image—even though Buffy and Olivia Benson find themselves in dangerous, violent sexual situations, they're never depicted as helpless.
The only really odd thing about this study is that 7th Heaven didn't seem to send anyone screaming from the room.