|Oh Father Ted, you never let me down.|
What does this have to do with RPGs? Well, a game that takes place entirely in the imagination has similar issues. There is the chasm between the mind of the creator and the mind of the user, and there is the chasm between the mind of the GM and the mind of the PC. In either instance, the Uncanny Chasm represents the gap between how it is "meant to be" and how it is "perceived to be".
Now, "meant to be" is a loaded phrase. Some will take issue with it, I'm sure, but at the end of the day, if something is created, that thing has a form. In order for you to actualize something, you have to conceptualize it, and in order to conceptualize it you have to visualize it. Basically, the game can inhabit some sort of quantum superposition where it has no direction, purpose or meaning if you like, but as soon as it's actualized, it collapses into a thing, and that thing has form, shape and function.
"Perceived to be" is much easier to to take in - it's the house rules, it's the home-brews, it's the on-the-fly judgement calls that make your game different from any other game out there, regardless of the rules you're using.
So how do you cross that chasm? How do you get others to play the game that is in your head?
Some will say, "Why would you want to?", and for them, I have no answer. To me, creation is an act of sharing, and if I'm going to actualize something, I see it as my duty to make sure that my vision is as clear as possible. So I don't create with these people in mind, and rarely spare them a thought - they will do what they will with what I create, or ignore it completely, as is their right. My goal is not to create some sort of totalitarian creative space where those who do not use what I have created exactly as I intend are herded into reeducation camps, but rather make sure that those who are looking for something full and alive are able to grok it with as little head scratching as possible. Basically, if you like what I'm selling, let me make sure you see it as clearly and fully as possible.
So, with that caveat out of the way, the short answer to my question, "How do you get others to play the game that is in your head?" is, "You can't".
No, seriously, you can't.
It is impossible to drag something down from the ether, define it completely, package it and hand it off to someone else without losing something along the way. Films probably come the closest, but even so, two people can watch the same movie, see the same pictures and hear the same words, and yet come away from the movie with different experiences and understandings. It only gets harder as you get deeper into the meatspace - two people can read the same book, and imagine the characters as completely different from how the author imagined, or even described, them. Once you get to roleplaying games, all bets are off.
So it's a matter of expectations - you certainly can't hope that someone is going to play your game exactly as you imagine it, and the world in the GM's mind translating well to the minds of the players is as likely as not.
And yet... and yet... it does happen. Rarely intentionally, but a thousands monkeys with a thousand typewriters and all that.
I don't think there is a simple answer here, only things that contribute in varying degrees to the ability of a project to bridge the chasm.
Some fall into the trap of thinking that it's the rules. They're partially right, but not in the way they think. I'll come back to that.
Art is an easier one to point to. It allows the reader to peek into the mind of the Creator. Problem is, unless the Creator is also the Artist, you've actually added another Chasm to be leapt. Art only gives the feel of a setting, though, not the setting itself. It can tell you what certain inhabitants and circumstances look like in the world, but it is by no means comprehensive, and it is almost impossible to communicate the "meaning" of a game via the art? Vibrant colors and strong, clean lines may indicate high fantasy, while drab coloring and heavy shadows may evoke noirish sensibilities, but there are other uses for each, as well - Superheroes use the former, while the latter could also be indicative of horror. So while not perfect, art does provide a viable, if a bit crumbly, bridge across the chasm.
Another way to go about it is with color. You know, the little bits of flavor text scattered through the book? They drop you into random parts of the world in question, giving you insight into the Creator's vision, without the investment of a novel. If you scatter them widely enough, your reader goes away with a good sense of things.
Then there's background. This one is tricky. Done well, it makes you ask questions, and draws you into the world. World of Darkness is a good example of this. While the conceptual element of their games are simple (Vampire, Werewolf, Mage, etc), it is legerdemain. Each game is its own rabbit hole, drawing you further and further from what you think you know about them. Done poorly, and you end up with an RPG that shall remain nameless. I grabbed the pdf off of DriveThruRPG, based on an intriguing blurb. Opened it up, and first had to scroll through a 48 page short story, followed by a 14 page history of this nightmare future, taken decade by decade from the present day all the way to the year 3,450 or whatever. All just to tell me that it is an Aliens-inspired Lovecraftian Survival Horror game. It took me all of about 2 pages to twig that, before my eyes rolled back in my skull and the tablet dropped from my hands.
Finally, we get to rules. I saved this for last, because it is such a fiercely contested topic, one which has given rise to two camps, which do battle regularly with the ferocity of a thousand tigers. Those camps are, of course, Team SYSTEM MATTERS and Team THE PLAY'S THE THING. I think that both are essentially correct, depending on how you approach gaming in general. If you want, you can take any rule set and use it to play any type of roleplaying game you can imagine. You can play a George Bernard Shaw-esque social comedy using basic Dungeons and Dragons. You can take the rule set for Classic World of Darkness and use it to play survival horror in space. You can bend, twist and house rule any system to make it do what you want, if you only want it enough.
Now, with that being said, there are some systems which make it easier for you to do this, and require less acrobatics on your part in order to make it work. For instance, while your characters may want to play an RPG version of The Importance of Being Earnest, you can either have them roleplay it out, in which case they have to possess the voice and vocabulary necessary to make it feel like an Oscar Wilde game, or make a lot of Charisma checks. But what if there was a system that was designed specifically to mimic social interaction of that nature? It allows those who enjoy that sort of thing to play along, without actually having to do it themselves.
While you may love OD&D, how many contortions must you perform to get it to work in a modern day setting? Clips of ammunition, the relative speed of a car vs. a horse, computers, etc. Can you do it? Sure, absolutely. You can figure out how to use those rules to make all of those things work. Or, you can use a system that is designed with specifically that in mind, and have it all there, no houseruling required.
So my point regarding rules would be, I think, that SYSTEM CAN HELP YOUR PLAYERS DO THEIR THING. There is no roleplaying game out there that can only be played by one system. You can always take your own approach and adapt some other system to do it. You can dig a ditch with a shovel, or you can dig it with your hands. Either one will get you what you want, but one is going to make it easier to get it done. In keeping with the theme of this post, you can build a bridge over the Chasm, or you can tie a rope.
Now that we're past that hurdle, it's worth it to mention that some systems make it easier to leap the chasm. Some systems operate in such broad strokes that the worlds described are instantly recognizable to almost anyone who plays them. I believe Dungeons and Dragons' initial success was due in no small part to the fact that the setting was generic enough that you didn't need too much background to be able to play. You say "Fantasy", and just about everybody thinks, "Dwarves, Elves, Magic, Dragons." There's no need to cross that chasm, because you're already on the same side. Pull out BECMI and start talking about the trenches of Verdun, and the Chasm widens between you and your players.
I don't really have any grand idea to wrap this all up with, there is no "solution" to the problem. There's a problem, and different things you can do to address it, but you'll never do away with it completely. As a GM, you will always care more about your world than your players will. As a designer, you will always care more about your world than GMs will. It's the way of the world - the Uncanny Chasm is littered with the scattered remains of tons of great ideas. While your ideas are more likely than not to end up there, there are things you can do to make it (slightly) less likely.