Sid Meier says "A game is a series of interesting choices." Taking this working definition forward, we can say that gameplay is a measure of the choices available in a given play environment.

Example 1 (interesting choice):
In D&D (third edition), crossbows have a threat range of 19-20, but their crit multiplier is only x2. Long- and shortbows have a threat range of 20, but their damage is multiplied by x3 on a successful critical hit.

Adding a third dimension to the choice, crossbows require a partial or full action to reload (depending on type), while bows can fire every round. At lower levels, this isn't an issue, but once a character has multiple attacks per round it becomes pivotal.

Example 2 (meaningless choice):
The first few rounds of Monopoly are a minefield of dumb choices. There is never a situation when you would not want to buy a property (until at least the mid game), and never a time when you can avoid paying rent when you land on a property. Coding an AI to play through the early game would be a trivial job.

This situation does not change until late in the middle game, when Go To Jail suddenly switches from penalty to blessing, and the unwritten rules of property trading and coalition-building begin to close the game.