Mappings introduce biases in the process of interpreting relationships between dimensions.
Some relations are much easier to perceive (e.g., data dimensions mapped to adjacent components) than others.
The accuracy with which humans perceive different graphical attributes varies tremendously.
Accuracy varies between individuals and for a single observer in different contexts.
Color perception is extremely sensitive to context.
Screen space and resolution is limited; too many glyphs leads to overlaps or very small glyphs;
Too many data dimensions can make it hard to discriminate individual dimensions.