This has no single answer, but here's my take.
Game design is behaviourist psychology 101: what behaviours do you want to encourage, what behaviours do you want to discourage? In more basic terms: what does the player do? What do they want?
- How do they interact, in the broadest terms, with "the game"?
- with their avatar (if any),
- with other players (if any)
- with the console / board / pieces (if any)
- etc
- What do the players want?
- To get points
- To defeat the opposition (human or otherwise)
- To complete the story
- etc.
The first is the "what" of playing the game, the second is the "why".
Continuing (the "how"),the rules and operations of the game can make any behaviour explicitly or implicitly
- Compulsory ("you must roll the dice each turn")
- Rewarded ("If you break the jar, get 10 rupees")
- Punished ("If you enter the room, the vicious beast attacks!")
- or Forbidden ("You may not move another player's piece")
Compulsions and prohibitions are heavy handed, and are in fact often implied in many games without being explicitly stated (and "presumed conventions of board games" are meat for another post). Rewards and punishments are the main link between what players do during the game and their pursuit of the goal.
The form of rewards and punishments, their relationship to each other (often typified by the relationship between risk and reward), to the final goals of the game, all form the backbone of the economy of the game, defining which resources are scarce, which plentiful, which can be exchanged, at what rate. This is true not only of baldly economic games, but any game which has resources and limits. There is an economy to the damage absorption capacity ("health points") of avatars in online shooting games relative to their speed and capacity to deliver damage which many games exploit for greater tactical interest.
And that relationship defines the meaning and the message of the game (of which more another time)
And that, to me, is the meat and potatoes, bare bones of game design. Picture what you want you players to be doing. Design a game that lets them do that, that encourages them to do that, that puts obstacles in the way of them not doing that.
Still, what do I know. Please, everybody tell me what's wrong with this picture.
