Add -ista to a word to connote "an aficionado of". Think "fashionista" and the like. The "a" feminizes the word (Spanish), and therefore, I guess, the term, which I have to say I hadn't heard before either. It seems to have been popularized by or made possible that book Doc cited, which, with the general puzzlement your thread has generated, tells me it's not a widespread term, and is still in the esoteric stage, at least to non-academics.
I can see why it came about, though, given the many pro-feminist themes of Buffy, especially. In 2005 I'm sure more than a few college papers in queer theory, pop culture, and women's studies had BTVS as their topic.