Theories:
- Han Solo not dead. Crawled under the dumpster.
- Chewbacca's shot really did a job on Kylo and he has to draw more than we realize on the force to stabilize his injuries, which is why he wasn't able to more easily swat Fin away.
- Rey may have had some early force training which was then wiped from her mind for her own protection when she was hidden on Jakku. This plus Kylo's injury was what allowed her to beat him.
- Fin is not related to anybody. Him being related to only other nearly age appropriate black guy in the Star Wars mythology would be ridiculous. Rey being related to Luke somehow is not as ridiculous if we believe that the Empire spent a couple of decades wiping out all the force sensitives they could find, meaning there are not very many of them left.
Luke just abandoning his child to that type of life though would be disheartening and it probably also means Ben turned a long time ago, at least 10 years, and that he is about a decade older than Rey because of her young age when she was left.
On the positive side, I hope Luke's daughter means we get some form of Mara Jade (who was human, not an alien as mentioned above) and not just a mention that she existed at some point and is now dead (possibly killed by Kylo because she was the one Jacen killed to complete his turn to the dark side in the EH - similar to Ben killing Han).
Another interesting thought on Rey's possible parentage that I've not heard yet is this (which would be another EU nod):
http://starwars.wikia.com/wiki/Lumiya