Kent’s rules, from Extreme Programming Explained are:

• Runs all the tests
• Has no duplicated logic. Be wary of hidden duplication like parallel class hierarchies
• States every intention important to the programmer
• Has the fewest possible classes and methods

In my experience, these don’t quite serve the needs of software design. My four rules might be that a well-designed system:

• is well-covered by passing tests.
• has no abstractions not directly needed by the program.
• has unambiguous behavior.
• requires the fewest number of concepts.