So, what is suspension of disbelief in games ? In my own words, suspension of disbelief is the ability of a gamer to accept something unrealistic or unbelievable in a game world and still immerge into it. Take a look at wiki for more details.
The tricky part for us game designers is, that you need to take care to not destroy this illusion. It is not as simple as just to say 'hey, he starts playing my game and he suspends his disbelief, so I've done everything right ! '. But more on this topic later.
Ok, there are two important things to remember:
1. Suspension of disbelief is a personal thing.
2. You need to ground your game world in realism !
The personal thing is simple. Make whatever you want and you will find enough people who can sink into your game without any problems. The second option is more important and will be your tool to control your audience. Everything, even high fantasy and sci-fi is grounded in realism. This is like a slider, on the left a purely abstract world, on the other side reality. A purely abstract world will alienate almost everybody.
Think about roguelike games, where an 'E' attacks the player, just an abstract letter. But on the other hand the description, actions, items etc. are taken from known fantasy worlds. There're many people who find it too abstract, still the roguelike community is huge.
An other good example are toons. Take a standard toon with animals in it. The animals talk like humans, movement and posture contains human features, they equip and use tools like humans, have human emotions and habits etc.
The designer wanted to create a world in which animals are the main characters, but the most important feature is, that almost only this fact is unrealistic. Almost everything else has a human touch, therefor it is grounded in realism we know and accept.
One of the best game ever contains one of the biggest misstakes in taking care of suspension of disbelief: Half-Life
Well, suspension of disbelief is a personal thing, but I belief that many gamers think in a similar way. Half-Life is about aliens, conspiration,experimental weapons,zombies etc. all about unrealistic and unbelievable things, still it was just a matter of
seconds to dive into this world. But, once you enter the alien world at the end of the game my suspension of disbelief broke. It was like a fist hitting you in the stomach. For me the game designers have gone too far, infact they stole my end-game experience.
So, what was the misstake, when suspension of disbelief is a personal thing ? Consistency ! They started in a realistic world, introduced the unrealistic part quite fast and hold this level for almost the whole game. But near the end they introduced the soldiers (in fact this is my remembered end-game), which is grounded in realism and then they throw you into ice-cold water, the alien world which needed a much higher level of suspension of disbelief then anything before.
If you want to address as many people as possible you need to went as far as possible to realism while keeping your game design, but never go too far with realism. Many genre already have some kind of template suspension of disbelief.
That is, the audience which chooses a certain genre accepts a core world, you could ground your game world in this genre world template. I.e. a vampire world , you don't need to explain the existience of vampires to draw your audience into your game, they most likely accepted this fact about your world after reading a title like 'vampire worlds'. Still you can explain it, but only for sake of storytelling and not so much to hold up the suspension of disbelief.
Finally you should prepare the expected level of suspension of disbelief quite early in the game and keep it on this level, as already said, a genre world could help a lot.