What capability truly defined the modern slot machine over its predecessors?

The ability to self-settle the transaction

The crucial demarcation point between rudimentary, manually-paid gambling devices and the modern slot machine invented by Fey lies in the ability of the machine to self-settle the transaction. This means the apparatus itself handled the entire process: accepting the wager, determining the outcome, and dispensing the reward autonomously without needing human intervention for payout. This mechanical self-sufficiency marked a significant evolution, analogous to moving from a machine that required a clerk to record results to a modern cash register that prints receipts automatically. This self-governing payout mechanism forged a new category of entertainment based on speed and reliable processing.