A good move has at least two ideas

@FireIslandGerbil because they had two kings on the board. its difficult to checkmate both in the same move. Especially when one of his king castles while the other does not

A good move has one to two ideas
1) Not getting checkmated
2) Checkmate if possible

@tpr Which is more important?

1) is more important: it makes no sense launching a checkmating attack on your opponent when he checkmates you one move faster. This is one reason why O-O is such a strong move: 1) it brings your king to a safer square and 2) it activates your rook. This also explains why king safety is that important, even more than the center, than development, than material. This also explains why in some attacking games the attacker makes a defensive move like Kb1.

@tpr, I think there are other reasons to castle. It is your only opportunity to move two pieces in one move for example, desirable from a solely macro perspective.

A good move has at least one idea.
-- steve

(Cannot stop thinking about O-O-O)

A good move must not have any idea.
Could be only an act of despair or lack of ideas.
--steve