Perspective, both Linear and Aerial.
Light and Shadow shows form and suggests depth.
A good composition is helpful to show off the painting to its best.
Scaling - show similar objects at different points in space so the viewer can estimate how deep a space they are looking into. This can be used with textures and details too, eg having windows in buildings so that the viewer can identify how large a human would be in comparison to the building; or having brickwork that gets smaller as it receeds into space.
Hope that helps
Digital Matte Painter // Forum Moderator