The main flaw I see is that his arms and legs appear out of proportion. You seem to be going for a mostly-anatomically-correct-but-still-anime-ish style here, but that clashes with the proportions of his arms and legs. His arms appear a little bit too long in comparison to his legs, and his legs also look too thin.
I actually think the hair looks pretty nice.
If you're worried about the shading on anything, I'd say the pants need work. They have a sort of gradient-shading going on right now, and I feel it's actually causing some problems with the way the pose reads. I understand the pose you're conveying, but it comes across strangely because the shading on the legs doesn't properly reflect it. His left leg is supposed to be forward I think, and the other leg back and behind, with his knee bent somewhat. The feet appear correctly placed, but either the size of the legs in comparison to one another is off, or the shading is creating the illusion that they are. I'm not quite sure which it is.
Of the poses you've shown, I think the 2nd looks best. The first one it appears the sword is just kind of balancing on his neck, and in the third, the sword appears far too large to be held that way. I agree with Alastor that if you go with the second pose, you should work on the elbow. That's the main element causing it to appear off.
If you want to consider another holding position altogether, might I suggest something like this?
http://www.thearma.org/essays/langk1.jpgI know the legs are in the wrong position, but I'm referring to the way of holding the blade. With a sword of that size, one of the few poses that would be comfortable is to rest the flat of the blade against the shoulder. This may look more natural than your other poses.
Hopefully I've made sense. You've got a good thing going on here, and it appears to be coming along nicely, but it needs a few tweaks.