Cain is going to wade into the minefield.
Has always viewed an RPG as a continuum. Especially lately when games are coming out that people say have RPG Mechanics (They never really define what those RPG mechanics are.)
Think of it as a checklist of game features. The more of them you have, the more Cain considers you an RPG. List has an order, but Cain will only give the order when he is done.
First one is, he can make his own character, including naming it. Very important, character creation choices should matter. They shouldn't be super limited, but they should be reflected on in the game. Your class, race, gender, attributes, skills, something in the game should be checking those. Gender could be as basic as changing if people refer to he or she, gender was a bigger deal in some of Cain's games but he thought it was ok as long as it balanced. Some people didn't like that. A lot of games put a lot of effort into character appearance, but if the game doesn't mention that Cain doesn't find it very important. Games of his that did have a lot of appearance features had that because someone else on the team pushed for them.
Once you are playing the character, Cain wants the player character to have a lot of choices on how to make. High level, how you decide to do combat/stealth/dialogue or have companions, or low level (What does your character does moment to moment., do you insult people or try to be nice.)
Third thing is that the story is reacting to your character selection (To a certain degree, doesn't want stuff you decided at the start to dictate too much about the rest) and action in the games.
Fourth thing is nonlinear story. Prefers to do things in the order he wants to do them, go where he wants to go, do what he wants to do. Repeats her prefers player driven story vs. story driven game. Story driven game is a narrative designer dictating to you what happens, doesn't like it. Wants the player to activate the narrative as they come across it.
Fifth, multiple endings. Cain expects different endings, dramatically different endings, ones with his companions and what happened to them.
Leading into story gating and area gating. Wants that based on character choices, not designer choices. Let's say you have a dungeon and you don't want the player going in there right away. Cain is fine with you implementing a key or passphrase the player needs to enter. He is even fine with you not spawning in the NPC the player needs to get the key/passphrase from until a certain point in the game or they are in a very hard to find place. What he doesn't like is if the door hard coded to never open until Act III. Cain has played games recently that he liked where he tried to step onto a continuation of the map and it tells him he can't go there yet because of his level or story act. Would be annoying if he went a door and it said you had to be level 10 before walking in. That is the developer slapping you, wants the restrictions embedded in the story.
Similarly, prefers for perks that have preqeqs be on a skill being a certain level or other perks purchased. Dislikes them being level gated. Is OK with them being gated into tiers and you have to buy a certain number of tier 1 perks to get tier 2 perks, which effectively guarantees some perks can't be taken until a certain level. Again, developer imposing on the player instead of the player choosing. This may sound narrow, but think of it as the player seeing choices they making opening up other options vs. developer dictate.
Wants the world to be big enough to support exploration and player choice, as well as adventures that aren't the main story.
That is Cain's checklist. Have all of those, you are an RPG. Take them off one by one, you become a little less of an RPG. Notice the order Cain went in, Systems -----> Story ----->Setting, the opposite of how he tells you to design games. But this is how game's present themselves most often. You are thrust into character creation, then you start playing and learn what kind of story they are being thrust into. Then as you play the game and begin to discover the setting. You may have heard a game blurb of the setting, but now you walk through it and discover it.
Many people disagree on what an RPG is. Some of it is down to how many things on Cain's checklist the game has. Some people say if it misses one it isn't an RPG, some people are fine with some of them being gone but others being gone are red lines. Some people treat it like Cain does, the more of them you have the more of an RPG you are, and vice versa. Some people weight them differently, maybe choices mattering is very important but making your own character isn't that important to them.
You could call them Hard RPGs, Soft RPGs, hardcore RPGs, casual RPGs, Cain considers it all a continuum. Cain doesn't think you should get too hung up on it, the continuum is rarely as set in stone as you think, references and links his D&D Koan. Making a hard line in the sand for RPGs is doomed to fail.