Oh, we're back to serious discussions now?
For me, it's structured decision-making within a narrative and character progression that makes an RPG. The less decisions a game has, the less effect those decisions have on the world, and the less control we have over our player character's progression, the less of an RPG a game is. Simple as that, in my opinion, but even then I'm not very strict with the term.
Fallout 4, even by my lenient prerequisites, certainly doesn't make much of an RPG. But I would hesitate to call it the worst RPG ever, or not an RPG at all. It certainly has decisions (very few compared to previous games), and they do have an impact (I try to say this without laughing), and there is character progression (not in a way that you could notice). But if we were to compare it to, say, New Vegas, then it would be left in the dust.
I think that regardless of one's definition of RPG, the one thing I believe we can agree on is that Fallout 4 lacks any and all of the spirit of 1 and 2.
By spirit I mean it's cRPG roots. Fallout 4 is a story that allows for some variation on how it is told but in the end the narrative allows for very little user input. Yes you can choose a path but none of those paths allows for any deviation on how you engage it or dictate its outcome.
There doesn't even seem to be a definite outcome. Fallout 4 goes for gameplay over story, and that's made clear. It feels almost surreal to see a game with cRPG roots having literally four video files for the endings, two for male characters and two for female. That is to say, pardon my phrasing,
absolutely fucking pathetic. This is spoken as someone who wasn't raised on RPGs (real ones) nor that big a fan of RPGs as other NMAers seem to be. But I came to Fallout for the exact same thing that isn't present in Fallout 4, and it took me quite some time to realise that.
but you won't find a valid reason based on what the game does or doesn't have (other than redeeming features) to claim that it isn't an RPG without culling many other good games from the genre as well. And that's how discussions about whether x or y game is or isn't an RPG always conclude.
There's nothing wrong with culling several good games from the genre of RPGs, as they don't make them any less of a good game. Far as I know, plenty of the NMAers enjoy Dark Souls 2 very much and some consider them their favourite games, yet term it as a "bad RPG". Then there's Morrowind, which was had unique visuals and a great exploration aspect, but it was still by most accounts here a "bad RPG". With myself, I'm quite loose with the term, but I can see what the old terms most NMAers go by represent.
Okay, you can affect quests and the lives of certain NPCs in many of these "RPGs", but the irrelevance of it all makes it seem rather pointless. The RPGness of it, so to speak, is apparent to me because I didn't grow up in the same period, but to older RPG fans, the ability to make meaningful choices in newer games are so small, I'm guessing it doesn't even register on their radar. I very much call Borderlands 2 an RPG in general discussion with other people outside this forum, yes, but by traditional terms, it is very much not an RPG. You do not have the ability to affect outcomes outright, though you have a fair degree of controlling character progression. This makes it an "action game with RPG elements" in that it's not an RPG but it does use certain mechanics present in RPGs.
I'm not actually sure what the definition of action-RPG is though, I'm guessing that's along the lines of The Witcher or Alpha Protocol, where it's more like RPG games with action elements.