For me I have shied away from a lot of feminist heavy debate because it almost feels like I'm an outsider, there are some posters who are very recognisable and it's almost like a online group-meeting for them, I'm not saying that's their fault! More of a lack of nerve on my part to enter into the fray.
I do think that healthy debate is dying a death in general and so more and more people resort to insults and condescension if you happen to disagree with them, if you make the stupid mistake of going against the grain on a thread you can even be subjected to a pile on for being the 'wrong' type of feminist (if that makes any sense?) so I tend to shy away from the topics in case I get on the wrong side of it again.
I think a lot of what people may think of feminists on Mumsnet is only lead by the strongest examples sometimes, in that, many level headed, intelligent and well-meaning posters who consider themselves feminists won't immediately register to other readers as feminists, but those who are prone to more provocative responses in a thread are easily remembered.
I could be completely wrong of course, but that's my opinion on the matter.