I think it's less those fans that grew up with the OT than those fans that extensively read the EU books. Of those I've discussed the movie with, that really seems to be a dividing line.
Maybe that is the dividing line.
In my memory of RotJ, Luke just rejected the dark side and converted Darth Vader (a mass murdering guy) back to the light side. He basically embraced his Jedi heritage fully. He's on a trajectory that suggests he will become a great Jedi and (as suggested by the title of the movie) the Jedi Order will return with him.
In TLJ, we learn that after that, we get a brief flashback and a talk about how he abandoned the force and briefly considered murdering his nephew, who was swaying toward the Dark Side (but as far as we know, did he murder anyone till then?).
The problem is that's plenty of character development that we don't actually get to see. We're just being told that major character changes occurred while we weren't looking.
Of course, they can't use Mark Hamill as Luke Skywalker to show us the movie we would need to make this character arc believable, because he's too old. And unfortunately, TFA already established that Luke was gone and Ben fell to the Dark Side as his pupil. Maybe TLJ's solution was really one of the best possible ideas to come up with under these circumstances. But sometimes, "best" doesn't actually mean good.
I guess that was one of the biggest challenges of TLJ - how do we deal with the open questions of TFA? It decided to focus on Skywalker's story, and shortcut most of the rest. Snoke, well, he's dead now (and had a silly name anyway), who cares how he got where he was? Rey's parents? Well, just some nameless scumbags that sold their daughter off. (Which while a bit disappointing as an answer, I think it's not that terrible - not everyone needs to have impressive or complex parents.)
But TFA obviously also made other big leaps in the story - like turning from a Rebellion that just won a big (and possibly final) victory over the Empire to an off-screen reinstatement of the Republic to a quick mention that the Republic is now gone again thanks to the Starkiller base.
I guess the fanservice is ultimately to blame for this - for the sake of the fans alone, they wanted to bring in the old actors in their old roles. But they needed a new conflict and new heroes to rise to the occasion, and that requires such big leaps in character and world development - but it comes at a price.
That should signify a strong presence of the Dark Side in her. Interesting angle, maybe? The movie never lingers on it for a moment, though. We see her do her 'trip to the Dark Side Cave' thing but it has no warning for her, just a riddle of her parentage.
I think the scene in the cave shows that the Dark Side can't offer her anything. She basically goes for the Dark Side (Cave) because it promises an answer. But instead, it gives her just a reflection of herself. Nothing that solves her questions. The Dark Side is just making empty promises to her. That might be what makes her "immune" to the temptations of the Dark Side.
Maybe. Maybe in the 3rd movie, things will change again.
I am afraid that the whole new Star Wars trilogy suffers also a bit from them not having a big game plan for the story of the 3 movies.