1) Simplicity-the design must be simple, both in implementation and interface. It is more important for the interface to be simple than the implementation.

2) Correctness-the design must be correct in all observable aspects. Incorrectness is simply not allowed.

3) Consistency-the design must not be inconsistent. A design is allowed to be slightly less simple and less complete to avoid inconsistency. Consistency is as important as correctness.

4) Completeness-the design must cover as many important situations as is practical. All reasonably expected cases must be covered. Simplicity is not allowed to overly reduce completeness.

www.jwz.org/doc/worse-is-better.html