Regarding the powers of Rey and Kylo:
I think if we look at how the force looked in the prequels, original trilogy, and now The Force Awakens, it's clear to me that the force becomes stronger and weaker depending on the epoch. In the original Star Wars, we clearly got a force in remission after a "golden age" of sorts seen in the Old Republic. Fast forward to The Force Awakens, and the force is all but non-existent with Vader and Palpatine killed, and only Luke--and apparently Snoke--to carry on the legacy.
Furthermore, a weak force season seems to coincide with a stark and strong imbalance. After Order 66 was executed, the only active force practitioners were Vader and Palpatine, while Luke, Obi Wan, and Yoda were all in some form of hiding. At the end of Return of The Jedi, only Luke is there to represent the light side. The force seems at odds with the wills of those who can wield its power in that there always should be light and dark, not only one or another.
Therefore, the force ebbs and flows, continuously distributing its power to balance out each side. Kylo Ren is powerful, but misguided in his mastery. Same goes for Rey, who is a quick learner, but doesn't understand her destiny.
I hope that made some kind of sense.