can you make the light shining in start at the bottom corners of the window sill? it just confused me initially and with that you would know where the light is coming from, since the sun shines in from overhead the light would be on the floor. really nice design though! flash?
the demographic this style typically appeals to tend to favour positive scenarios, so you might be pitching to a tough crowd with this. this could really benefit from a positive twist. which is easier said than done but there you go.
i would agree with jshicks that the light shining in is a major distraction right now. get that fixed first. then i'd say add some texture to the outside elements (tree & sun) to set them apart from the girl and her "inside" world.
I see why the light is as far up as it is - to make it clearer you could add a suggestion of floor with an oval or a few well placed lines/arcs to give it the 3D perspective it needs.
Btw - love the personality of your sea monster shirt!