Question: are you using lightmass? If so, read through the lightmass page, especially the translucent shadow sharpness. http://udn.epicgames.com/Three/Lightmass.html
You seem to be missing the mark for your 3D models on all the things that make your concepts cool - the sharp shapes, the personality, and the proportions.
i would be carefull with the sharp points on the legs. even if its going to be normal maped it will look weird in the silhouette. post some wireframes.
Hey that's much improved. Never be afraid of contrast and highlights. My only crit is maybe what you're alluding to, with the lack of sharpness in the vents.