Normal mapped models do not always look like plastic. If you ever created your own normal mapped models then you will know that you can control the metallic / plastic look with the specularity map. If there is no specularity then it would even look very dull and not plastic.

When you look into the latest showcase shots of my textures then you will see some parts that have more specularity than other parts. If you use that wisely then you can have a quite realistic look. And there is no shader to blame.

And Matt's statement with the flat color maps is right. You have to create them flat otherwise it will look wrong in the engine. You cannot paint a shadow in the color map and after that the real-time shadow will add to that. Often they can look contrary to each other.

Color maps should have only: colors. Lighting and highlights will come through normal and specularity maps. That is what 3d artists did in still render images for several years. And now it comes in real-time.