Okay, so I noticed something that is actually a bit of a running theme in the Star Wars universe, and that is this whole Dark vs. Light thing. Luke made a mistake when training a new batch of Jedi by sensing Snoke's attempt to turn Ben Solo and somehow thinking that he had already fallen. Since Ben was so powerful he had this instinctual reaction to cut him down in his sleep, but Ben woke up and saw that Luke looked about to kill him and did the only thing he could do: fight back. Ben didn't know that Luke had come to his senses just before that and wasn't about to kill him.
Does this sound familiar? It should, because it's basically the same thing that happened with Anakin in the prequel movies. Anakin was shown to be very powerful, but the Jedi Order didn't want to even train him in the first place. Extraordinary circumstances saw him actually getting trained, but they always were wary of him, kept him at arms length, didn't promote him as he likely should have been, which allowed Palpatine to get into his head and sway him over. They sensed things in him that unsettled the Order, and so they sought to lessen his influence and strangle his power. This just caused him to lash out; rubber band towards Palpatine and the Dark Side.
Now let's move on to Luke and his snap decision to kill Ben Solo. He sensed the Dark Side in him, but instead of talking with him and trying to reason, he reacted much like an animal would: kill that which might be dangerous to him. Don't forget, Ben Solo is his own nephew, too. This is all because Luke never actually completed his training with Yoda way back in Empire Strikes Back. Think back for a moment to when Han and the others went to Cloud City to hide out: Luke sensed the trap that they walked into, and fearing for his friends decided that he would go and save them. Yoda warned him that he should stay and complete his training, but Luke wouldn't listen. As it turns out, Luke being there made no difference, and he paid for this by losing his right hand.
Remember during Luke's training when he entered that forest that was filled with the Dark Side? Yoda warned him then as well to not enter it, but enter he did. He faced who he thought was Darth Vader and after a brief fight lopped off his head, only to find right after that that it was Luke's own face under the mask. This was to allude to two things: that Vader was some relation to him (which he learned later in Cloud City) and that even Luke has the Dark Side in him. I imagine that this was a lesson that Yoda only learned in his isolation on Dagoba: that it's truly impossible to not have any darkness within yourself without turning yourself into an unfeeling droid. Luke left before he could learn this lesson, though.
Now, cut to the new film. Rey sought Luke to bring him back to the Resistance, and be trained by him. Luke, as it turns out, has cut himself off from the Force for many, many years, and refuses to train Rey. He watches her as she trains herself, and decides to give her three lessons. On the first lesson, though, he finds that she can sense the pit on the island that contains much Dark Side within it, and although he acknowledges that it's there to create balance, he also recoils because she apparently didn't resist the Dark Side calling to her at all. Then later he finds that she had some kind of connection to Kylo Ren, something that also disturbs him, and puts a stop to it. Later, when Rey is getting frustrated because Luke didn't tell her the truth about him and Ben Solo the first time, she travels to the aforementioned pit to try and get answers concerning her parents, and finds nothing there; no answers, no influences, just a mirror surface that seems to go on forever.
That last part is telling. Rey has gotten basically no training whatsoever, and has no preconceptions about the Force. As such, while she can sense the Dark Side, it isn't influencing her like people who are already biased tend to be. She's similarly called to the tree that contains the Jedi Scriptures. Perhaps Rey doesn't contain a perfect balance in the Force, but it's a whole heck of a lot better than every other Force user that we've ever seen. The Jedi were always trying to keep perfect serenity and shun baser emotions, the Sith were always about reveling in emotions and taking everything beyond the limit no matter the consequences. The Jedi never learned that you cannot just cut your darkness out of yourself, while the Sith understand that everyone has both Light and Dark in them, they just ignore that in favor of gathering more power for themselves. At best the Jedi suppress their Dark Sides to the point of it more or less exploding, and either they wind up sacrificing themselves or they fall.
So there, that's my spiel. The Jedi never learned their lesson when they still had power and numbers on their side, and the Sith never learned enough restraint to keep their power and prevent the rebellion that brought them down time and again. Had either taken to heart that keeping balanced even with bias towards one side or the other is the best policy, they could have won a decisive victory and much of what happened would never have happened.
I think that's all from me for now. I might choose to go watch it again to see if there was anything I missed.