The key of the solution of a chess problem is considered (in our times) that is not good when is true anything of the following:
If the key is obvious.
If there is no defense of the white for a certain check of the black, this is called unprovided check and, since it must be stopped, it reveals the key.
If there are not set mates for certain flights of the black king in a two-mover, then the key becomes obvious.
If the key takes a piece of the opponent.
Taking a piece is very “fierce” and aggressive key and is not good. (If we see such a key in an orthodox problem, it will have probably humorous content). Taking a pawn is tolerated if the problem is generally interesting.
If the key is check.
The “problem with checking key” was common in previous times, but today it is not desirable. It can be accepted only if it is impossible to compose a “problem without checking key” showing the same theme or the same task.
For heterodox problems this condition is not valid. For example, it is quite acceptable in a Helpmate problem to have a checking key.
If the key does not allow the opponent to give check.
If the key limits the mobility of the black king (in orthodox problems).
[This Post in Greek language]