Sorry I'm late to this party, but I think what Dervel said upthread could do with reiterating - Joss as good as sacked the actress who played 'Cordelia' for the crime of getting pregnant. Whilst she was pregnant he called her 'fat'. He didn't just write her out for the duration of her maternity leave he destroyed her character in a way that people are still angry about 14 years later. Cordelia had been in his shows for 6 years and that was how he treated her.
All his shows are problematic when it comes to women (I love Buffy, it's a fantastic show, but it's not feminist - unless feminism is about being able to punch the hardest) 'Angel' (t.v home of the aforementioned Cordelia) is just depressing in its treatment of women (in 5 years it only has 3 female regular cast members (in contrast to 7 male) 2 of those die of mystical pregnancy (the 3rd is a comic relief vampire who is only regular cast for the last few episodes) and 5 other mystical pregnancies appear throughout the show - 2 of them given to Cordelia (the 3rd one killed her) who was then fired for getting pregnant in real life!
Firefly -don't even get me started on Inara! (in the future the greatest honour a woman can hold is being an academy trained prostitute) and if you want to see something that will really make you vomit then just read up on Tim Minnear's episode that (thankfully) never got filmed where Inara is gang raped by reavers but when Mal finds her he cleans her up and I quote - 'treats her like a lady'.
Then there is River Tam, the damaged, violated, slightly crazy genius - who actually has a lot in common with Fred - the damaged, violated, slightly crazy genius in Angel (who makes up the third and final mystical pregnancy death in that show.)
In Doctor Horrible, Penny is the only named female character, she doesn't interact with any other women and she is fridged (admittedly right at the end.)
I haven't seen his Marvel work, but I have been informed that Black widow describes herself as monster for being barren and claims that they sterilise their female assassins because it makes killing easier for them. Unless there is a whole lot of dialogue around this that somehow negates that sentiment entirely, that seems pretty offensive a statement to women in general and infertile women in particular.
So, yeah - his feminist credentials have long been overblown. If you actually look at his work there is some really troubling stuff in it. Buffy was hailed as feminist because it had a well rounded (she gets to talk about boys and demons) female protagonist who had a female support network. That this was seen as extraordinary doesn't make Buffy feminist - Its just a shocking indictment of the rest of television.
A lot of people don't like hearing this though because they really love Buffy and they get very defensive over any criticism (unless its a criticism of specific episodes). But even Buffy should not be above criticism.
Thing is, if you want hilarious lines, great acting and more heart break than the human soul can safely endure, then Buffy is the place to go. If you want feminism though - you need FWR 