I think the reason everyone thinks she's a man is that there really is no defining female features on her. The breasts are small (not that the have to be huge, just noticeable), her hair could pass as a guy's hair, mouth and eyes look like your generic anime "cool" guy, the weapons are anything but feminine (I love the huge sword though. I want one). One thing you could do is slim down her limbs. The arm in the front is rather thick for a woman, and the hands could be more shapely. Another thing would be to get the fact that she has breasts across. That's usually the most noticeable feature in which we seperate men from women.
Oh and probably the biggest thing is the pose. It's a pretty common pose that are usually used on male characters. A lot of power, but no grace, which would show it's a girl. | 
