It is more important for the interface to be simple than the implementation. Correctness-the design must be correct in all observable aspects. Incorrectness is simply not allowed. 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.
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. I believe most people would agree that these are good characteristics. Simplicity-the design must be simple, both in implementation and interface.