A few thoughts.
1. Re: the Force and how it works
To be fair, we really don't know all that much about it, at least based solely on the films themselves. In the original three films, the Force was a mystical energy field, created by life itself, which surrounds, penetrates, and binds the galaxy together. It controls your actions, but it also obeys your commands. Some people have a stronger or weaker connection to the Force, although how and why is never made clear. Through the Force, you can physically manipulate objects without actually touching them, and size is less important than believing in your abilities (as demonstrated by Yoda lifting the X-wing from the swamp). You can also use the Force to jump really high, to anticipate the action of your enemies, to "mind trick" people, to choke people, and to shoot lightning from your fingertips. And that's about it.
In the PT, that gets expanded to super-speed, super-enhanced reflexes, and various acrobatic feats, but the explicit powers themselves don't really change much. We just see them on display more. What really changes is the explanation for why someone is stronger or weaker in the Force, namely the midichlorian count in their cells (which can be determined through a blood sample). Apparently, the higher one's midichlorian count, the stronger one's connection with the Force, leading us to believe that it is the midichlorians themselves which interact with the Force. Midichlorians are also necessary for life itself, and can communicate the "will of the Force" to people who have them in sufficient numbers. But, again, the rest of the info is fairly vague on the subject.
The Clone Wars cartoon apparently dealt with concepts regarding two different "aspects" of the Force: the Living Force, and the Cosmic Force. The Cosmic Force is the aspect which binds everything together, and is fed by the Living Force. The Living Force was the aspect of the Force that was created by life itself. Through manipulation of the Living and Cosmic Force, one could appear as a Force ghost/spirit. In a metatextual sense, this strikes me as the Clone Wars' attempt to fuse what seem like two very distinct notions of what the Force "is" into a single, unified theory that more closely approximates what we learned about in the OT, without totally throwing out the PT concepts. But, regardless, this is canon now.
What we still don't know is just how independent a (ahem) force the Force actually is. The whole "controls your actions" thing implies that the Force can actually make people do this or that...sort of. This could explain how -- out of nowhere -- Rey is suddenly able to use the mind trick, use the Force to help her fight Kylo Ren, etc. Of course, we still don't know why this would happen, what the Force "wants" exactly, or whether the Force truly "wants" anything the way we do, or simply naturally seeks to establish some kind of balance in the universe which has somehow been upended.
I would bet that, in addition to the whole "Living Force" and "Cosmic Force," there will be some facet of the Force that relates to emotion as well as to intention. And, of course, one fantastic way to expand the story is to simply say that what characters said previously about how it all works...was simply wrong, or incomplete. So, the Old Jedi and Sith both had incomplete understandings of the Force, and neither way was truly the "right" way.
I have my own theory that there's this underlying thread in both the PT and the OT (and it's one of, in my opinion, the most interesting things that the PT really introduced, even if this was unintentional), which is the idea that emotion -- and specifically, emotion in balance -- is the essence of life itself. If the Force is created by all living things, then to cut oneself off from emotion is to ultimately deny oneself true life. But the flipside of that is that passion -- while an incredibly strong connection to life itself -- can ultimately lead a person to do destructive things (either self-destructive or destructive towards others). So, too much emotion will ultimately overwhelm life and end it.
If this is the case, then what Luke started (and Rey will presumably finish) is the process of moving the Jedi to an order which is capable of feeling deep emotions, without letting themselves just wantonly indulge those emotions. "Light" and "Dark" are less about the emotions themselves, and more about how those emotions manifest and the intentionality of the individual.
2. Re: Kylo Ren as a villain
I think basically that Kylo Ren is a more modern take on the villain. Instead of just being some cackling, moustache-twisting, power-hungry guy who does things purely for self-serving interests, he's someone who is motivated by a twisted sense of what the greater good is, and is willing to do awful things to attain that, even at the cost of his own soul.
In a way, he's a much more dangerous figure because he's a "true believer" in his cause, and has a "by any means necessary" attitude. He's not amoral or a vicious sadist. He's a crusader. To me, that's way, way more dangerous. A self-serving guy you can deal with. A crusader, a fanatic, there's just no controlling.
