Hay guise!
I'm no expert but here are my two cents:
Everything needs moar contrast, in particular the guy. He's way too bright, specially the shadows.
You need to make the floor tiles less regular, because if you can actually see the grid. Tiles shouldn't show any features that line up with the actual grid - it breaks the illusion.
On the artsy side, some frilly stuff (roots, tufts of grass) where the trees meet the ground would really be worth the effort - just a couple of details here and there and, voilą, spiffy tree.
Very good for a first timer, though. Real horrorshow
Keep up the good work. DO polish this, it has the potential for awesome!