My therapist asked me once, "What does it mean to you to "be a man""?
I couldn't really answer him. I went to a couple boards I was on, and couldn't get a good answer there, either.
Strength? Leadership? Honesty? Willing to stand for what is right? Those are things I'd expect from a woman, as well...
But, if a male yields to another male (without legal reason, ie, to a commander in the .mil) he's seen as "weak" or "like a woman." A man can be spineless--culturally, this is more severe than if a woman is seen as spineless, because he loses the respect of other men (I don't know how women see another woman who is spineless)
Women are seen as more nurturing; men are seen as more distant and "cold".
Again, those are cultural definitions.
I know testosterone fuels aggression--so it could be argued that one element of "manhood" is violence, and the (self- ) control of it. If their propensity to violence is used for good (defending the weaker (women, children, etc) that is a good thing, if they use it to harm others for their own gain it's bad. This violent tendency is helped by the fact that men have, in general, a stronger physique than women.
I don't know, just random thoughts on the subject.