The design must be simple, both in implementation and interface. It is more important for the interface to be simple than the implementation. Incorrectness is simply not allowed. 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.
All reasonably expected cases must be covered. Simplicity is not allowed to overly reduce completeness. I believe most people would agree that these are good characteristics. The design must be simple, both in implementation and interface. It is more important for the implementation to be simple than the interface.