Love the idea, this could turn out great

Right now, I wouldn't say this has much of an expression...It kind of looks a bit like ÒoÓ, but not quite.
Is there a reason why you've cropped it the way you have? I think it would be nicer seeing more of the tree. At the moment, it's not immediately obvious that it's a tree. It takes a few seconds to register that it is actually supposed to be a tree.
I don't like the foreground raindrops...they do nothing for the piece and make it look as though you've accidentally gone over those pants with an eraser tool. What you could do instead is add more depth to the bg...possibly a hint of an horizon and/or silhouettes of other trees or rocks in the bg.
Ryu had some good advice on mixing ramps. Because of your seperate palettes, the rock doesn't seem to really fit in the scene. Also, you could try making a yellow from the tree a more yellow-green colour and use that instead as the leaf highlight
Just a suggestion: maybe you could add some small bugs or something around the tree. Could add some more life to the scene and give the tree a bit more character.
Also, Maybe darken the scene to give it more of a stormy vibe.