Aside from what Pype addressed... Well, actually, I'd like to bring up some points to support and elaborate on his critique.
In the lineup of trees, the first two trees' trunks look a bit flat. Personally I'd prefer something along the lines of the third tree. You seem to be using about 7 or 8 unnecessary colors on the trees alone, and about 5 on the character sprite. And I have to ask, the light source is coming from the upper right/overhead, yet the darkest spot of the tree (as indicated by where the rimlights begin, or where the lighter purple leaves start to show) is at the lower right, where it should be at the lower left/bottom, so why not put it there? If I were you, though, I would ditch the rimlights altogether.
Quick edit on the tree, character, and a quick GUI mockup:
Other things to note, the values on the grass--namely in the large dark patches surrounding the zombies--are a lot lighter and less contrasting than those on the trees, which I find a bit odd. They also share no common colors, almost making them seem as if they came from two different sources.
Right now the grass looks like a sea of green clouds (albeit well-done ones). I agree with Pype in that they could use a bit of texture work.
The character sprite's left arm doesn't seem to abide to the isometric nature you've set up.
Gotta hand it to you though, excellent job on texturing the trees.
EDIT: I was hoping you'd understand.
At least you get the point of the edit?
