I think it depends of the player, really. There are different "groups" which have different expectations.
Games like "The Sims" were an enormous success because they put girls to videogames (I have 2 sisters, I know what I'm speaking of :lol

. The gameplay is based on the evolution of one or several characters. (edit : and they introduced psychology in videogames, too).
FPS are a classic genre. They provide quick, instant action for players who like to "aim and shoot". Perfect for quick de-stress after a day of hard work, for exemple. The gameplay is based on putting the player in a combat or dangerous situation (adrenaline).
Simulation is more for people who like complex things. I belong to them, and probably a lot of people around there do so. I would say that SimCity4 and Orbiter are both simulators. The gameplay is about modeling as closely as possible the reality focusing on a subject (town building, space travel & exploration).
They are many other genres, and of course they are often crossed (some (MMO)RPG games cross character developpement, FPS combat, exploration...).
I would add humor, which really make games better, and can be added to any genre (example The Sims, "Beneath a Steel Sky" (for the old-timers), and so on...)
edit2 : some "super-uMMu v9.0" could introduce customizing astronauts, adding experience points simulating specialization (endurance, dexterity, piloting skills, stability of the behaviour under stress...), that would somehow cross The Sims & Orbiter

... I think the only limit is imagination !
