Because of this, I think it's important to agree on definitions of terms in the context that you're discussing them, so that subjectivity doesn't become a problem. These glossary posts should help clarify some of the more specific terms I employ when discussing game design.
I'm not suggesting that these are neccessarily the "right" way to define these terms, but they are the ones that work best for me. I find it's much less important that a definition be correct than that everyone in a discussion have the same definition in mind.
For any efficiency, the tendency of increasing costs to be less effective at increasing rewards. Diminishing returns may only apply above a certain cost level, or they may scale over the entire range of possible costs.
The ratio between the cost of an action and the perceived value of its reward.
1) Assuming something to be an absolute truth when it is only true in specific cases.
2) Mistaking correlation for causaulity as a result of such faulty logic
The usage of metaphors to present game mechanics and gameplay as a cohesive and logical whole.
All stimuli the game presents a player with, in the hopes of encouraging or discouraging certain gameplay.
Anything that a player does or thinks as a direct or indirect result of a game's mechanics.
A game's plot or story. This includes the conflict, specific characters and their arcs, the backstory of the world, and the happy ending.
The amount of rewards generated in a particular span of time, regardless of how efficiently those rewards were generated or how much effort and resources they cost.
Productivity = (Effort x Efficiency)/Diminishing returns