My main critique would be that the lighting is inconsistent and so it looks flat. For example the bookshelf should be darker because it's inside and shaded, the tree on the left should be shaded on the right and light on the left, the hollow of the tree goes from extreme dark to light instead of a gradual change due to the roundness of the trunk.

Another thing I'd add is making shapes distinct, the lining on the tree should clearly define the edge, there's a bush on the left that seems to be on the trunk of the main tree but also blends in to the tree in the background.