I have a novel in progress with two male protagonists. At one point I considered the possibility of making one of them female. One of the characters is the "straight man" of the piece and the other is more colorful--he has a good heart, but he's egotistical, obnoxious and amoral.

I realized I could make the first character female, but not the second. Because while people will tolerate--if not exactly like--obnoxious egotism in a male character, they detest it in a female character. I don't know why! It isn't fair! But I've found it's much harder to create interesting female characters than interesting male characters, because many traits that are accepted in men are despised in women. I wish it wasn't so.