Your main problem in all these scenes which makes them look pretty bad is lighting. Try using an external renderer, or at least using some ambient occlusion in your scenes, it really helps to make things look more realistic, instead of having to use a hemisphere lamp to prevent shadows from being completely pitch black. Also, using the default specular settings and no bump maps decrease the quality a bit, since a texture which suggests contours with a completely flat specular and diffuse overlay just doesn't look right.