I've already been tainted by some spoilers, but I'll speculate on a few things I don't know so much about.
1) The knights of Ren are being manipulated by a Sith Lord. I don't think Kylo will be a Vader-level villain... I think he wants to be, but he's a kid playing dress up compared to a real Sith Lord. The actual threat is someone else.
2) Rey is Luke's daughter, Kylo is Han and Leia's son. I can't explain why I think this exactly but I believe its important for there to be another Skywalker (not just a "Solo") and Rey is the prime candidate. Luke may be in exile because he broke his vows by falling in love, or maybe Rey's mother was killed and he was tempted by the dark side so put himself in isolation to overcome that darkness. While its possible Rey and Kylo are siblings, I think its more likely they are cousins... the sibling thing was done already in the Old Trilogy. And it just makes sense that Han and Leia would have a child.
3) The Force Awakens implies a return to a more mythological presentation of the Force (i.e. not a scientific one, "midichlorians"). Speaking of the "will of the Force" and of destiny is an easy way to explain the sorts of chance meetings and coincidences that will bring our heroes together.
4) Leia is not a Jedi and did not receive Jedi training (judging from the trailer she seems to have taken more of a New Republic leadership role). But she does know where Luke is (maybe she is the only one who knows).
