I was hoping, as good communicators do, to rephrase it to see if I understand it, and then build on it.
So, the core idea that started Travis rolling was the tendency of role-playing hobbyists to call the rule sets that they tend to use "systems." But, if you consider what a system is, the game rules alone simply don't fit the definition. After all, the rule set is only an engine under which a lot of other things that go into the game runs on. And, if you consider how things are played in the Free Kriegspiel Revolution, then the rules may be almost unnecessary to role play.
The Impact of Setting Facts on Play
In the FKR, they say that you play the world, not the system. Whenever possible, players or the referee or both consider what makes logical sense given the fact established in play or the development sessions that built the world in the first place.
If, for example, you've established that the elves draw heavily on the darker aspect of Celtic lore as I have done in Xen, they will require blessed weapons, black Iron weapons, or possibly magic to kill them. In any other scenario, an elf might be overpowered and captured by a significantly more powerful opponent, but can only be completely removed from the story through a very narrow scope of means. That means when an elf meets a knight on the battlefield, or a giant, the results need to be interpreted in that context, never mind the rules. In an FKR game, it would probably be established at the knight would die a valiant death trying to take on an Elven swordsman, while the elf might end up trussed up for a century of torment before breaking free from a Giant's captivity unless rescued later.
Those facts are every bit as important to the game as how an attack roll is resolved. They are just as much a part of the system.
Player Expertise As System Element
Likewise, consider the open elements of the game where player expertise has an impact on the possibilities within the game. Here's a great example of that:
My player characters in my Xen campaign are currently trying to acquire the teeth of a guardian Naga for a sorcerer who in return will make them a magic item. In that temple, the player characters find the ancient offering chamber which depicts several gods who were shoehorned into a Pantheon in the early days of the Empire in order to create stability. Supplicants had to crawl through an eetily decorated crawl space on their hands and knees and arrive in a pool of holy water to kneel before icons of the gods in order to offer their treasure.
This pool was surrounded by serpents made of marble that were, in fact, real serpents magically petrified by cockatrices. A spell glyph carefully hidden on the statue of the Bodhisattva Maiur would cast stone to flesh on each of the serpents should anyone steal the treasure.
My player characters were exceedingly cautious in this ancient treasure chamber. Their painstaking search allowed the party thief, Reina, to discover the glyphs, and they guessed what they would do.
One of the few magic items the party possesses is a jug of alchemy. This item can produce up to ten gallons of any fluid that would not hurt The jug itself up to 10 times a day. However, only one liquid can be named each day.
Because one of my players minored in chemistry, he was able to tell us that nitric acid would break down the bonds and marble, but not harm the glass that the bottle was made of.
They melted the statue before robbing the tomb and got away with 6,000gp, one magic ring, one cursed ring, a sack of 29 gemstones, and a suit of +1 bronze plate
Permeability of Rules Systems
And, for that matter, as an inveterate OSR hacker, my rules engine is quite malleable. I am using Treasure Types and Encounter tables from AD&D when I. In a hurry. I use the Monstrous Manual from AD&D2e when I want a stat block not in Swords & Wizardry, and I have adapted firearm rules from Lamentations of the Flame Princess. I have mountains of material incorporated from The Dozen Dooms as well.
At any given moment, I can grab by Low Fantasy Gaming, LotFP, AD&D 1e rulebooks, the Monstrous Manual, my Rules Cyclopedia, Swords & Wizardry, Deathtrap Lite, Knave, or DCC RPG and adapt content or a rule as I need to. My guiding principle being "Would it have felt out if place at a table in 1979?"
My current Xen ruleset is Swords & Wizardry, plus Oversix, plus a collection of established house rules, plus whatever I need from 75% of the TTRPG content on my shelf. Is my rule set even an identifiable system?
Certainly if anyone asks, I say I am running Swords & Wizardry crossbred with Deathtrap Lite and a few add-on rules.
Game System as Ecosystem
TTRPGs are extremely open-ended experience. Their narrative structure means that the total parts going into a campaign can't easily be enumerated. It's like Canada's constitution: we know we have one, but it is so big, complex, and has so many inputs nobody can clearly delineate what is and is not a part of it.
If anything, I would suggest that RPG "Systems" really are each, as the FKR suggests, our particular to the campaign because they include both the closed system of the mechanical rules and the externalities of player knowledge, table culture, the fiction of the world, and who knows how many other incalculables.
And perhaps that is one of the things that makes it so enjoyable as a game. No two campaigns will be the same. Even if you were to play strictly RAW with a game system, and randomly generate as much as you possibly could (or use only modules), the particular imprints of the table and the players at it would make it unique and ephemeral.
Role-playing games are not a system in the way a Super Nintendo is a system. They're more like an ecosystem, complex, ever evolving, delicate, and prone to massive change over small adjustments.
The mechanical rules are possibly one of the smallest parts of that.
Extra-Textual Play Cues
This is also, by the way, why I don't discount the importance of Art in role-playing games. Art creates tone and context when you consider the art in the rule book, it helps create the culture of the game.
I would say that a great deal of the success of Dungeon Crawl Classics RPG, and the reason people find it so Gonzo is as much the artwork as it is the craziest results from the spell tables. People who play Dungeon Crawl Classics tend to play it in a certain over the top way, which the text of the game doesn't necessarily demand or support all the way through. However, the artwork, the tone of the adventure modules, and the culture they have built around themselves certainly does.
And the players who are attracted to DCC are probably predisposed to Gonzo play from the start and found the look and feel of the manual resonated with their disposition. Or put another way, the Art says "look at this crazy, gonzo pulpfest!" And the players looking for crazy, gonzo pulpfest are drawn in and bring the friends they know will dig it. And they bring the crazy and gonzo to the table.
How Understanding the Nature of the System Can Help Run the Game
I spent a considerable amount of my academic career trying to understand the discipline of Cybernetics, which is the study of systems of human knowledge and behavior. TTRPGs from a considered using the tools of Cybernetics can be fascinating... But also dull, academic, and full of excessive jargon to explain "no duh" evident phenomena at the game table.
But one of the insights it can give me is this. Every change the composition of the table, every new tool, artwork, or idea added can change the entire landscape of the campaign. Your choices about what to introduce, which NBC's to create, and what themes to explore radically change the entire campaign each time they do so.
Each game is a complex of memes that work together to make it possible to create a narrative virtual reality experience, that can collapse very quickly, or be sustained for years depending on what is added, when and how.
It is hard to clearly articulate every possible variable that goes into creating a game and making it fun, memorable, and long-lasting. Generally speaking, a GM who plays enough games, and journals about them, will end up with an intuitive understanding of how to cultivate the game in the direction they want.
A lot of the advice in the OSR blogosphere is built in that direction. How to build a campaign that lasts. How to build a game that captures imaginations.
The best bloggers seem to have a strong grasp of what will and won't contribute to a fun, and during campaign. In fact, that is the brilliance of Gary Gygax' AD&D Dungeon Master's Guide; he gives you at least a surface-level picture as to why he has chosen the rules he has in order to cultivate a particular type of game. If anything, the biggest problem with the AD&D DMG was that it was incomplete. Gygax assumed that people would have enough of a background in wargaming, for example, I do understand exactly what a "campaign" is without a definition. The definitions added in later editions failed to hit some vital ideas.
At the end of the day, what we can learn when we start thinking of Dungeons & Dragons and other TTRPGs as a system is that they're open to cultivation and shaping. What we add into them can have a radical effect on how the game plays out. To the point where no two campaigns will ever look alike. If you set out mindfully to cultivate a game in a particular way, not just with story elements, but with characters, artwork, a selection of your players, the music being played, the stated goal of the campaign, etc, are all as much a part of the game as the rule set. And can be chosen within tent.