It's important that SuperTuxKart has a consistent object style throughout. In many ways, this page is analogous to the texture guidelines.
Objects in SuperTuxKart should be realistic in size, shape, general form. However, objects should not be as detailed as in real life. Take a palm tree for example:
You can see that the the new palm tree model far more closely matches the shape of a real palm tree. However, it is not so detailed that it looks real.