The Three Big Questions
1) What is your game about?
2) How does it go about that?
3) What does your game reward/discourage/encourage?
What is your game about? – PREMISE
Your game needs to be about something. It can be leveling up a character to face greater challenges, it could be about the stress of working in a business environment, it could be about protecting people unable/unwilling to protect themselves. But it needs to be about something. I call this the premise.
How does it go about that? – ELEMENTS
The game (mechanics, setting, everything) should all work toward establishing and reinforcing that premise. If your game is about leveling up, make that MEAN SOMETHING. If the game is about tragic romance, have rules for flirtation and for breaking up with people. Elements are all the pieces of your game.
What does it reward? – SYSTEM
Rewards drive play. This is the “engine” of the game. D&D uses XP’s as a base (because XP’s are used to level the character) but it also uses money, equipment and context (better tactics, new allies, enemies, knowledge, etc.). The System is how the Elements fit together to establish and reinforce the Premise. This is the core of Game Design.
These things build up onto one another and become more focused as you go along in the process:
Why do we play?
What is it?
How does it work?
Answer these and your game will work. Fail to answer them and it won't.