So, I watched this video and it brings also a definition at the beginning (of a long and interesting journey throught RPG-history). In the definitions it mentions the things we in the thread and I in my attempt mentioned, but one more, and I could smack myself for not getting it: world building. Sure, not every RPG has good world building, but so has not every RPG deep statistical mechanics or a deep story. But if I think of RPGs, many of them have detailed worlds and that is a thing many non-RPGs actually lack.
So if you're interested, here is the video:
https://www.youtube.com/watch?v=8o3i10OuMFQ