Hope this edit helps:
 
---MINE --- YOURS

I did some minor tweak in his left hand and his right arm. You draw the hand beneath the arm, so it looked like he was freezing.
That's the only problem I saw with the position in comparison to the reference.
Someone might help you a little more. 

Here's an edit I made (Note: I'm no pro, not even close). But you need to look more to your reference, and if you don't have one, google it. Here's is the one I used: link.

And here is the incomplete edit.

- The shape of the one you made looks very weird.
- The skull you made (something I didn't edit) has three central incisors.
- The skull is looking to the right, but we can see the skull's left cheek/jaw looking frontwards.
- The eye sockets are too triangular.
- The highlight is very strong and seems unnatural. (Mine isn't perfect, I know, is pretty awful  , but maybe you'll get the idea)

And finally as twodayslate said, finish the skull, then start adding details/accessories.

I don't think that is a problem. Trees in Maple Story don't happen to be friendly.

What I've notice is that the concept, even without the leaves looks more like a tree. The other one, because of the roots and, maybe the colours, looks more like a rock (in comparison to other graphics in the game). But as it is a work in progress, I don't think that is much of a deal. I'm sure that when you add the leaves that "rock" feeling will go away.

Right now it looks very good. When I saw it, I thought, this reminds me a lot of Maple Story, then I finish reading your comment

Any ideas?

Those branches looks extremly sharp. Branches don't end like that, look: old tree.
The base also looks strange.

Google some trees, that will help you a lot.