The most annoying thing I have seen since seeing the very exciting news that Avatar: Legend of Korra (which I seriously hope will not be called The Last Airbender: Legend of Korra) is all the people questioning what gender Korra is.[link]
And she's buff. That doesn't automatically make it impossible to determine what gender she is.
Avatar Kiyoshi was buff. So was avatar Yangchen. It comes with the job. You can't be Avatar without being in excellent physical condition.
And it's not like the girls in the original series weren't also athletic but still feminine. I don't get where people are getting this idea that she looks like a boy because of her physique. Or maybe it's her physique combined with the clothes? Nah, can't be. Katara did skirts. So did Suki. Ty Lee had pants. Azula had pants. So it's not as if the animation team has confined girls to "traditional" female apparel either.