Suspension of disbelief: It refers to the willingness of a person to accept as true the premises of a work of fiction, even if they are fantastic or impossible. It also refers to the willingness of the audience to overlook the limitations of a medium, so that these do not interfere with the acceptance of those premises. (Wikipedia)
How thick is the wool you're willing to pull over your eyes? The tolerance level for suspension of disbelief varies greatly from person to person, and as far as the general audience for games go their willingness to suspend their disbelief is much higher than the average non-gamer. I don't think many developers and gamers even realize how far they're willing to go to force their own immersion into a game. Consequently, this affects their designs and limits their audience.
We need to make it so that the amount of suspension needed to be immersed into a game is less than it is now. For gaming I define suspension of disbelief somewhat differently. As gaming is concerned, suspension of disbelief is about masking your mechanics to a point where they are no longer considered systemic mechanics, but merely an extension of the world you are inhabiting. Let me give you an example to clarify what I mean.
In Left4Dead when a player goes to heal another player with a medpack they don't think to themselves, "I'm going to heal Francis because he's down to 12 health, and this medpack will heal him for 75 health bringing him to 87 health, which will increase his run speed as long as he doesn't get hurt again." They think, "Shit! Francis is hurt, I'll patch him up." The less we get players thinking the first way and the more we have them thinking the latter way, the better.
On the other end of the scale, there are mechanically heavy games like Warhammer Online, where in almost all of your choices are based on your knowledge of the game as a system. It quickly becomes, "I'll choose this piece of loot for the +14 weapon skill over this one with +14 initiative." Obviously, there is a demographic who enjoys that sort of nuts and bolts gameplay that involves getting into the nitty gritty of the game system. Diablo 2 thrived off of it.
What I'm saying is that if we want to open our games up to the non-gaming population more we need to implement our mechanics in such a way that they aren't smacking us upside the head with percentages. A game that did a great job of making the nitty gritty mechanics more natural is The Witcher. In The Witcher there is plenty of old fasioned RPG nuts and bolts, but the game makes the compromise (improvement) of not forcing the player to learn their entire system in order to play the game. Instead, I can simply choose to add another dot onto a skill whenever I level up, and instead of describing it to me as "+25% damage per skill point" it is described as "Sinew of Blows" or some such. Leveling the skill then unlocks a new animation that I'll see Geralt perform when I attack next using that skill, making me feel like the character himself has improved and not just his statistics. The descriptor coupled with the new animations that leveling up unlocks makes what would traiditionally be considered a classic skill tree in a more natural playable direction. It's not perfect, but it's trying.
If we can lighten up that wool over the players eyes, we can get more and more people playing our stuff and enjoying it for what it is as a whole piece, not just power-gamers playing it as a mechanical system. Remember, just because you don't mind having your character described as a robot with a number assigned to each skill, doesn't mean the player won't mind.