Okay, so all games are simulations of some sort.
When I say "simulation-based", others may say "rogue-like", or "strategy/sim". Such games usually have the following characteristics: procedurally generated maps, dynamic story generation, character growth, and the ability to a degree for the game to 'play itself'. Every game does at least some of what I've listed. But I believe a simulation game must implement all these characteristics.
Over the next few months (years?) hopefully I can share some of what I've learned as I attempt to develop just such a game. The genre has always fascinated me since the days of the early CRPGs and sims. While I enjoyed arcade games, they never inspired me to develop games. But there was something about games like the single-player Ultimas, SimCity and Civilization that sparked my interest in game development.
So what makes a game more "simulation-based" than, say "story-based"? Where does the FPS fit in?
Procedurally generated maps
While one can play on pre-generated maps, most of the time it's played on a map generated and populated at game start. Examples are dungeons in Rogue/Nethack/Diablo. Terrain maps in SimCity (1-3) and Civilization are other examples.
Dynamic story generation
Where the game builds the story in response to player actions, versus a primarily pre-scripted experience. Story here means a series of events/outcomes. The story is not pre-determined except possibly as a broad set of goals (to rule the world, the kill the foozle, etc.) But the player can accomplish these goals through a variety of means - this is the player's 'story'.
Character building and progression
It is a game after all, versus a weather or traffic simulation for example. Also, the simulation reacts to character progression, advancing the (dynamic) story. Perhaps every game, platformers to RPG alike has this feature.
The Ability to Play Itself (to a degree)
Events (i.e. story) occur independent of the player's influence. The player may affect story, but it should be possible for the game to run without regular input from a player.
In strategy games like Civilization, this means that while you may be playing the Americans, the Aztecs may have declared war on the English, conquered them and built a dozen cities before the player has even met either the Aztecs or the English (or anyone else the've been in contact with.) In a simulation like SimCity, one can a city running for hours, days, or longer without any input from the player. The simulation drives the story.
Contrast that with a traditional single-player RPG like Dragon Age where the story doesn't progress until a player completes a series of quests - the player drives the story.
So... that was a good summary. Next time, some details on procedural map generation!