Lots of beauty and potential, lots of problems too!
The sun in really inconsistent: with the light on the scene (rather soft and from the top), with colors (that yellow doesn't fit too well, also the sun is not yellow!), and with the surrounding sky (the sky is usually very bright around the sun).
The tree is outlined on the right side only.
The grey border on top of the clouds isn't necessary, I think.
I actually really like those clouds a lot. The crates look a little flat compared to the rest. The bottom tree looks completely excellent, but it looks really weird where it goes up and blends with the upper tree. Some tile edges are a little too abrupt, too. Other than that, holy crap this looks great 
EDIT: water and mountains look nifty too, I thought I'd add. I think the sun is fine as it is.
looks pretty good, but the crates, clouds, and definitely the sun needs work.
Dear god I love those trees.
Obligatory crits: Colors are sort of all over the map. You could rough up the leaves on the right side of the trees' foliage a bit more. Also, the crates don't have outlines...it looks odd.
Very clean, but the sun is pillow shaded. Try anti-aliasing instead of pillow shading?
I've been inconsistant but I've had very little time to fix it. I'll amend this according to whats been said by when I have the time
Thanks for the comments.