Great question. In 30+ years of gaming, I've had women in just about every gaming group I've been part of. In my current group, the women outnumber the men. But now that I think about it, over all that time I've only played in a single, one-shot adventure that was run by a woman.
I know my wife feels she doesn't have sufficient rules mastery to run a game (despite 10+ years playing). I keep telling her that's not true, and that it doesn't matter anyway, but she hasn't hit the proper desire/confidence ratio to run. Is that a common issue?