If you wanna make it look interesting/weathered, I'd say remove a few shingles here and there and maybe making sections of the door look like wood, to suggest missing paint. The plaster(?) on the outside could have some cracks going through it here and there...The windows could be cracked or have a kind of frostiness around the edges.
If you don't want to make it look old, maybe you could increase the size of the shingles by a little bit so that the top isn't too crowded, and offset them from one another a bit. As they are, they look more like bricks than roofing. Make the edges where the roof comes to the sides overhang a bit more, or stagger the shingles there. As the others said, some roof-windows would be nice.
The grass, as Andy said, should be less contrasty and have more compact patches.
The tree's looking cool.