What Really Makes a Kick-Ass Heroine?

Okay...morning post was a little tidbit to whet your appetite for A Witch In Time. Now I've had a bit of a breather from the morning crazies (we got new furniture today - after ten years with Mr. Xandra, we finally have a Real Bed (TM) to sleep in--woo hoo!), I'd like to talk chewing bubblegum and kicking ass. Well, mostly the kicking ass part, since, as the saying goes, I'm all outta bubblegum.

What makes a kickass heroine? What made Buffy so appealing to so many people everywhere (and do you prefer the movie!Buffy or the series!Buffy - Whedon opinions notwithstanding)? Who here is a Xena fan? Who cheered out loud at a con when the news came through that Voyager's captain was going to be a woman?

I've thought about this (and had several discussions/idea trades/fandom_wanks over it) - what is it that makes a heroine kickass? And if she loses/sacrifices her femininity to do it, why? Does femininity=ability to be a dishrag? What does being a kickass heroine mean to the hero?

I have noticed a subtle, almost unconscious defensiveness...a shock and awe, if you will, among many men, and not a few women, when confronted with strong female characters, who are strong in the sense that male characters are usually strong. When you say "strong female character," people can think of a wide range of female roles going from Steel Magnolias strong women (emotionally stable, sensitively supportive, etc.) to Xena (can lift a Roman chariot and throw it at someone). But when confronted with the Xena end of the spectrum, there's invariably charges of she-maleness thrown about, and people's comfort zones get pushed.

Who's your favorite kick-ass heroine? And why is she kick-ass?