First we have to ask: "What is an RPG theory?"
If someone could explain that in simple words, that would already be very helpful,
To me, "RPG Theory" is trying to explain RPGs in order to help design new RPGs. Not from a game mechanic perspective (though theory can help with figuring out why mechanics aren't working or aren't fun) but from a larger perspective - what makes people want to play RPGs? Why do they work? What makes them fun? What can you see if you look across lots of different RPGs to see where they're similar? Where are they different? Why are they different? Do those differences matter? If they do, why do they matter? etc.
Despite my complaints about GNS and the terminology used with it, I do think that it's got a good core of observations to it for game designers to consider. At the core of GNS is what Edwards termed
agendas but which I think are better termed
motivations for players - why are players sitting down to play a particular game? What are they trying to get out of the game when they sit down to play it? Why are they coming back to the table to play it again? What about the game is engaging the player?
Edwards very broadly defined three motivations for players (that's the GNS). Since this is a + thread I'll leave my snarkiness about naming these motivations aside, but I think they're a good place to start when trying to explain why a game engages its players:
Type 1 is about being motivated by challenges - the thrill of victory and the agony of defeat. If engaging with the rules of the game and through clever play and/or good luck achieving victory brings you to the table, that's a type 1 motivation - which is the "G" in GNS.
Type 2 is the one that I think is the fuzziest - it's all about exploring some aspect (or multiple aspects) of the game for the joy of exploring that aspect, not necessarily in the service of beating a challenge. This can be as varied as wanting to play "in character", or exploring the setting as a world, or even playing with the mechanics of the game to see what you can do with them. All of these end up lumped into this second type of motivation, which is the "S" in GNS.
Type 3 is about story - wanting to have a game with a satisfying narrative. A beginning, a middle and an end and have it all "make sense". That's the "N" in GNS.
One mistake that was made in the original GNS model was to put these motivations in opposition - I would argue that most people who enjoy RPGs have at least 2 of these 3 motivations to keep coming back to the table, if not all three. A game that only satisfies one of these motivations is going to have a smaller pool of players who are interested in it, while a game that satisfies multiple of these motivations will have a broader pool of players interested in it.
For instance - part of why I think that D&D is so broadly popular is because you can use it to satisfy all three of these motivations. The mechanics of D&D are good at scratching that "type 1" itch, while the structure of the game is open enough that the "type 2" motivation can be satisfied without special mechanics, and if you have players who are interested in having that satisfying narrative the game can be adapted to allow for that as well - as the plethora of Actual Play streams and podcasts demonstrate.
I think it also helps to explain why, contrary to so many opinions on the Internet and yet empirically supported by evidence, D&D is actually a great introduction to RPGs. Because folks who are new to role playing and come from other games are going to naturally have a lot of "type 1" motivation to learn new games - overcoming challenges is pretty much what non-RPGs are all about. So D&D provides a platform to broaden the idea of what a game can be in a way that is gentle to new players - unlike RPGs that downplay that type 1 motivation in favor of either enforcing a narrative structure or relying on players wanting to explore their characters/world/system. I would argue that D&D
teaches gamers what an RPG is in a way that other RPGs just .. don't. It provides a satisfying "game loop" for folks expecting a game to be about overcoming a challenge while also providing the flexibility to learn to explore in the game and also teaching players to game with a story in mind.
I would also argue that if you look at RPGs with this motivational lens in mind you can see how D&D (and it's offshoots) truly are different from other RPGs on the market. From my POV most traditional RPGs that are not D&D focus very heavily on satisfying that "type 2" feeling - whether you're talking Call of Cthulhu or Traveller or Vampire the Masquerade the draw of the game is to exist as a character in a world and explore what that means. The actual challenges in the game that are overcome through clever play and luck are often secondary to exploring what it's like to be a character in the world.
Now you can play D&D that way - many people do - but it's hard to do the reverse and get a satisfying game experience out of it. Why are we playing Call of Cthulhu if we're not getting into character and exploring that Lovecraftian world? Why are we playing Vampire if we're not going to get into character as an angsty immortal pseudo-teenager with superpowers? The point of these games is often much more to explore the setting or the drama of being a character in that setting than it is to setup a challenge and knock it down - in fact often the published scenarios for these sorts of games the challenges are just ways to set up opportunities for players to do that kind of exploration.