Shankar Vedantam had an interesting piece in the Washington Post called "The Myth of the Iron Lady" that offers a theory about why female leaders are so often made to sound mannish.
As he points out, if you look through some entries on Wikipedia, you'll find out that Margaret Thatcher "was called 'Attila the Hen.' Golda Meir, Israel's first female prime minister, was 'the only man in the Cabinet.' Richard Nixon called Indira Gandhi, India's first female prime minister, 'the old witch.' And Angela Merkel, the current chancellor of Germany, has been dubbed 'The Iron Frau.'"
It's not news to point out that many people refer to female leaders as "mannish" because they feel uncomfortable with the idea of women having power to begin with. But that's not Vedantam's only point. Rather, he suggests that it might be more a result of a psychological zero-sum game. After pointing out that in our culture, women are considered the "nicer sex," that men are considered more aggressive, and that we tend to associate leadership with stereotypically masculine characteristics, he goes on to suggest the following:
"Experiments show that women vying for leadership roles are automatically assigned two labels. The first is to be seen as nice and warm, but incompetent; the second is to be seen as competent but unpleasant. Women stuck with Label A cannot be leaders, because the stereotype of leadership is incompatible with incompetence. Women who do become leaders get stuck with Label B, because if leadership is unconsciously associated with manliness, cognitive consistency requires that female leaders be stripped of the caring qualities normally associated with women."
So, in other words, a woman can't be thought of as "womanly" if she wants to be perceived as a leader, because female characteristics are, unfortunately, still often associated in people's minds with incompetence. Therefore, if a woman succeeds at being a leader, she must also be accused of being mannish -- otherwise, her success just doesn't make sense.
I think it's interesting to consider the idea that we might jump to call female leaders "mannish" out of a psychological need to resolve cognitive dissonance -- regardless of what characteristics the women actually exhibit. Of course, I don't think that's the entire story -- most powerful leaders actually do exhibit characteristics like strength and decisiveness, because those are characteristics that are required of good leaders. The problem is that since these "leadership" traits also happen to be associated with masculinity, we've essentially equated "masculinity" with "good leader." It'll be a great day when so-called feminine characteristics -- grace, sensitivity, kindness -- are also equated with leadership (or, for that matter, when we stop assigning genders to our abstract nouns), but I'm not holding my breath.