But in reality, all these overly perfect characters do is create more unrealistic goals for girls. If someone wanted to be a true radical feminist, they would create a character with all the strength, brains, and ability of someone like Buffy, but with none of the physical attractiveness. Think about how empowering that would be. To know that, as a woman, it is possibly to be powerful and successful without being the prettiest girl out there. Joss Whedon, and many other writers often set out to subvert gender stereotypes and create well rounded characters for their audiences enjoyment, yet none has been radical enough to subvert the one stereotype that has been plaguing womanhood since the first man hit puberty. A woman's worth has always been tied up in her sexuality, but to truly move beyond this point of contention, someone has to create a a Mary Sue, who just happens to be ugly. Then maybe, just maybe, the feminist writers would have nothing negative to say.
Article about Mary Sues on TV Tropes.com. See how many of our heroines fit these traits.