How fun the game was at the cost of a credit.
If the game was not fun enough to justify spending the nickel/quarter to play, then it was a bad game.
The really good games were known in my area as the "quarter guzzlers." Because they were extremely fun, but still hard, meaning you wanted to keep playing at the cost of the quarter or credit per play. And you'd end up dumping multiple dollars into the machine before you knew it.