Any ideas on how I could improve this?

Him being the only green thing within a warm toned wall bugs me a bit. normally, you would have something else of the same color in the bg to support his green coloring. as well, he has no shading? and shouldn't he be a silhouette if he's standing/posing in front of the sun? ;P okay, i'm picking on strange things.

anyway, the meshed texture would look cooler if it were not throughout the entire wall...and i think your sky's okay (although i'd say lightly burn tool some of the stripes towards the border of the wall just to get some variation) but your ground's the part that's not so good. the horizon part is...odd, can't put my finger on exactly why...although some of it is brighter than the actual sun. =/

not many nice ideas here but *shrugs* i've got to run. i'll check back later. =]