I think that the pixelated look on the canopy kind of clashes with with the detail on the rest of the piece.
Also, at least on my monitor the bright side of the root look like it is made of a single color which doesn't look good. (on closer examination I noticed that there are 2 colors there)
I whipped up a very quick edit:
I just changed that hard to see color a bit so it will be easier to see the detail. (it also makes the trunk a bit more consistent with the lighting on the canopy imo)
I also broke a few of the pixel blocks in the center so you can see how the canopy looks without the "simplified, textured feel", also in a very simple manner just to get a feel of what it would look like.
As for shadow and grass (what you asked for feedback on); I think the grass is quite nice, it does a nice job of conveying "grass" without using many colors and is very "silent" so it does not take the attention away from the focus of the piece (the tree), the same is true for the background. The shadow however... It looks to me like it doesn't exactly conform to the shape of the canopy, I suggest you decide exactly where your light source is (in terms of angle from the ground) and just draw lines to see where the shadow from the canopy's edges should land and draw the shadow accordingly, currently my instincts say that the shadow is drawn as if the light source is at 30 degree angle and the lighting on the canopy suggests more of a 40 degree angle.
But maybe that's just me
(I think the first thing you should do with any criticism is try and see whether you think it is correct
)
EDIT: Is it just me or did the palette change in more than one color in my edit? :X I wonder what I did wrong :X
Anyway, just try and change what I changed in my edit and see if you like it better, shouldn't take long at all :S
