> knowing how to do shadows based on a light source
Admittedly not everybody has the same level of visuospatial ability, but generally it is a matter of thinking of the angle at which light strikes the surface.
For example, virtual every GUI widget I've ever seen is drawn as if the light is coming from the top-left corner, so surfaces facing top-left are the brightest, while surfaces facing bottom-right are the darkest.
Admittedly not everybody has the same level of visuospatial ability, but generally it is a matter of thinking of the angle at which light strikes the surface.
For example, virtual every GUI widget I've ever seen is drawn as if the light is coming from the top-left corner, so surfaces facing top-left are the brightest, while surfaces facing bottom-right are the darkest.