My tastes in terms of genre are pretty conventional: mostly fantasy, a bit of sci-fi, a touch of contemporary.
The RPGs I mostly play are dungeon-crawling D&D (my house-ruled variant of the c 1978 version), scene-frame-y D&D (the 2008 version), PbtA-ish Traveller (my modest variation on the 1977 version), Prince Valiant (1989), Burning Wheel (I mostly use the 2005 version, with a few elements from later versions ported in) and Torchbearer 2e (2021). None of this is very radical; it's only the rhetoric of some posters that sometimes makes me feel otherwise.
I don't know if I have a precise sense of what counts as a RPG. I do have a reasonable sense of what makes for a fairly conventional RPG:
*Central to play is the creation and the change, over the course of play, of a shared fiction;
*This creation doesn't take place in a "writers' room" fashion - rather, there are distinct and asymmetric roles for the participants;
*The GM has a special responsibility for the backstory/setting, and draws on this to present fictional situations to the players;
*The players have a special responsibility for particular characters, who are present within the situations the GM presents, and who respond as described/declared by their players;
*The players' declared actions generate changes to the shared fiction.
*This creation doesn't take place in a "writers' room" fashion - rather, there are distinct and asymmetric roles for the participants;
*The GM has a special responsibility for the backstory/setting, and draws on this to present fictional situations to the players;
*The players have a special responsibility for particular characters, who are present within the situations the GM presents, and who respond as described/declared by their players;
*The players' declared actions generate changes to the shared fiction.
The different ways of doing this are what makes for a system.
Most often, I see "system" used to refer to published rules text - so D&D 5e, or AD&D, or 1981 Traveller, etc, all count as "systems". And secondarily, I see it used to refer to some components of an action resolution method - mostly, the basic framework for characterising difficulties, together with how to generate a dice roll result (eg d% roll low against a fixed skill number; or roll, add a bonus, and reach a target number; etc). When used in this way, the principles that govern the setting of the difficulty, the consequences of the check, and the like, are not normally included as part of the system.
But when I think of system in an analytic/explanatory context, I think of the method - the procedures, principles, heuristics, mechanics, etc - used to work out the content of the shared fiction, and how it changes. At a minimum, system includes the GM's methods for undertaking and using prep; how the ingame situation is established, and presented to the players; how actions are declared by players, and what counts as a permissible action declaration; and how consequences of actions are established.
For me, RPGing is about the shared fiction and how it changes. And in a conventional RPG, that's all about the relationship between framing of scenes/presentation of situations (by the GM), declaration of actions (by the players, for their PCs) and what follows from those declared actions (consequences and outcomes).
So the more the GM controls of all this - where, as I've posted upthread (making the analogy to other games, like chess and bridge), control is not the same as authority - the more of a railroad it is: the GM is controlling the shared fiction (perhaps in response to player prompts). The converse of this is player agency - the players exercising control over the shared fiction. The asymmetric roles in the game mean that the GM will always have some control, particularly via framing and some aspects of outcome/consequence. But when the GM has all of it, or most of it - eg all the players are doing is to declare what actions their PCs take, but all the rest is with the GM - then that's what I call a railroad.
