Okay, first of all, this picture caught my eye straight off the bat. The thumbnail just drew me in.

Now, for the most part, your porportions look fine. There's not much to nit-pick at there, but there are some things that are really bothering me. First is his head. With the angle we're seeing, the amount of face that's showing, we would see less of the side of the head, if that makes any sense. It makes the head seem too wide.

Also, where the wrist meets the hand, it looks like it bends unnaturally, down then up. It's not too noticable, but little details really bother me...

Ah, next, his hair is all one shade except for where the smoke goes over it, with a couple strands that catch the light. I think that you should add some more shading, maybe some darker and lighter tones to it.

Some of the folds on his clothes... you should make them seem to fall naturally. Maybe even set some cloth down to look at how the folds go. Right now, it looks like his clothes could really use a good ironing.

And I think that's all I'm going to pick at. Beyond those little problems, the piece turned out great, it really did. I faved it almost instantly.