I am also contending that mainstream games have a highly specific culture of play, expectations, and set of play techniques that most of do not normally look at with a critical lens because they represent the default of what most of us consider a role playing game to be.
I'm not trying to brow-beat you into changing your choice of wording, but I'm going to have to work may way through this, every time, because I can't imagine I'll ever be comfortable calling RPGs 'mainstream,' in any sense or context.
What you're talking about is D&D, and I suppose d20, and related/similar games, when they are Played A Certain Way, that being some variation on the ways that folks traditionally settled on to get D&D working for them back in the day. That said Way Of Playing is relatively monolithic, and used by such a large majority that it's relatively un-examined, from within. And, that this de-facto majority constitutes an 'in group,' with indie games - really, any game not D&Dish-enough - on the outside. From which outsider perspective, you are looking in.
A "there's two kinds of gamers..." proposition.
I don't really quite agree with that way of looking at it. Rather, I see the community as having a lot of D&D-only to D&D-primary participants who mostly may, indeed, have the sort of in-group perspective you're positing, and then various much smaller demographics who mostly have also or even do still play D&D, but have much greater exposure to one or more other significantly different games, including some who are very cosmopolitan in that regard (like, I'm guessing, you, pemerton, & manbearcat, just for instances). The D&D-only/primary crowd is by far the largest, since D&D is the only RPG with significant mainstream name recognition (in the actual mainstream of society), so where the majority of would-be gamers start - if they can't stand it, they may not ever find out there's a lot of alternatives, and that's it, they never really join the hobby; if they like it (or learn to live with it, at least), they join the D&D/d20-centric majority ('mainstream' in your terminology, I'd almost have to say cult, since we're talking the core of a fringe-sub-culture that has endured decades of relative obscurity prior to the current come-back), if not, they go looking for other games and fall into admiration of one or a few of them settling into a 'niche,' or eventually become more cosmopolitan.
Here is my basic contention: The different expectations, culture of play, and specific play techniques in utilized in game like Sorcerer provides an experience that does not easily arise when playing modern Dungeons and Dragons. The same is true for Moldvay B/X. although modern D&D can come closer there. This is even seen in when Imaro criticizes indie games from the prism of that culture. It's all about the experience the GM provides and satisfying individual kicks as seen through the prism of Robin Laws' Player Types. Story as seen as something the GM provides.
Well, The
DM Provides.
I don't much care for the Forge conclusion that games have to somehow force everyone who uses them to Play A Certain Way or else the game is 'incoherent,' nor that a game that a game having chosen an agenda to 'support' must do so by blocking or punishing others.
Frankly, I think a game could do well to be fairly open to being played in a variety of different ways, and that a well-designed game that's robustly balanced will naturally tend that way...
Here's why I find this analysis flawed: It is done with zero reference to basic features of the culture of play that makes Sorcerer the game that it is. When I play Sorcerer I am not looking to the GM to provide me with an experience or a story. We are all exploring these characters together. I am not just invested in my character. I am also invested in what everyone else brings to the table, the definition of Humanity we worked on together, and finding out how human our characters are. I would be very interested in going into more detail about this culture of play if there is real interest.
Note: I never meant to imply that mainstream games were less fun than OSR and Indie games.
I think most of us would have less fun with mainstream games - like monopoly, for instance. ;P
Though, seriously, you don't need to mean to imply it in a comparison like that, the implication is going to be seen by & antagonize anyone even a little defensive about their place (of 'privilege' even) in the hobby's dominant segment.
I only meant to convey that they are not somehow contained within and represent a narrowing of the basic experience of playing a role playing game. Difference of kind. Not a narrowing of experience.
If we're drawing set diagrams, there's a 'universe' of people and an itty-bitty circle for the set of people who actually play TTRPGs. It's heavily overlapped by larger circles - science fiction fans, people you read comic books, fans of My Little Pony, MMO gamers, CCG Gamers, etc. It's entirely contained in the broader 'Gamer' set, even though there are some aberrant individuals who play TTRPGs without ever touching a video game, let alone play poker for money....
While the actual mainstream largely thinks "D&D" is the whole hobby, we know that TTRPGs are a whole category with many quite different games. I think it's more important that any two given RPGs (even if one of them will almost always be D&D), are both TTRPGs, and overlap in the experience they provide, than that they're different in what they provide or how. Though there's certainly value to people realizing there's more out there in the rest of the hobby than they may have yet had personal experience with.
As to the stuff at the top:
I’m perfectly fine with “(system-)constrained GMing.” I would include system in there because I’m not sure that is implied to your average RPG conversant (first question may be “by what?”).
As for “fiat GMing”, I’m absolutely fine with an alternative.
How 'bout "DMing" and you can have "GMing"
I just find that folks on ENWorld typically connote “unconstrained authority” over the gamestate and over the disposition of play (with caveats of distasteful behavior and abject malfeasance of course) as SOP for the discipline of GMing. And there is plenty of support for that position with various iterations of Rule 0, “lead storyteller”, “sole arbiter/referee”, “GM’s game/setting”, and the other aspects of system which require heavy GM mediation (and authorize it). Tony uses “empowered GMing.” I’m not sure that carries any sort of differentiating accuracy
It's just what the current ed of D&D is calling the 'Golden Rule'/'Rule 0'/Illusionism/the-Killer-to-Monty-Haul spectrum of DM Disorders/Variants/House-Rules/Improv/Covert-Freestyle-RP/etc. "DM Empowerment."
And I'm consciously using DM rather than GM, because there's plenty of games that don't count on the reality that the GM can do whatever he wants, but actually try to present workable systems, even if they might not always be used.
(in fact, I personally feel less empowered as a system piles mental overhead and increased resolution mechanics mediation upon me). So what is a descriptor that differentiates upon the spectrum of authority/latitude/constraint? “Apex latitude GMing.” That sounds so terrible that it just_might_work... (not really).
Just as the 5e Fighter's "Best at fighting" is 'best' in the Advertising Claim sense of "no alternative has been conclusively proven to be strictly better," DM 'Empowerment' is 'Empowering' in the management-fad sense, of giving you more responsibility, maybe a snazzier title, but no additional authority or pay. ;P