Symbol design can be a very challenging visual communication problem that involves many of the human-use issues encountered in product design.
In many cases the symbol must convey ideas rather than literal translations of physical objects. Additionally, symbol design requires an understanding of graphic design.
The symbol must have appropriate semantics. The visual image must convey a message. Understandability, cultural biases, age demographics, existing standards, learnability, and inadvertent interpretations all must be considered when assessing a symbol's semantics.
The symbol must have appropriate syntax. The symbol should have good visual properties. It should look nice! Elements of the symbol should relate to each other, and the symbol should also fit into its intended environment. The symbol construction should be consistent in its use of figure/ground, solid/outline, color, etc. The most important elements should be recognized first.
Finally, the symbol must be usable. Can the user see the sign under anticipated conditions: lighting; distance; viewing angle; or other visual noise? Is the symbol vulnerable to vandalism or difficult to reproduce? How does the symbol scale?
Wow! This makes me want to design a symbol!