Rey was too much of a Swiss army knife. Mechanics, self defence, flight operations, languages, control of the force... Like others have said, things just came way too easily for her. It's one thing to be focused and determined, quite another to just start controlling people and objects with your mind without any doubt or frustration. The whole relationship between Fin and Rey was a bit over the top. Would've been cool if she had dissed the Falcan by calling it a "piece of junk" to echo Luke rather than just "garbage".
I think the implication is supposed to be that Rey HAD some training already, she was just so young and it was so long ago she either forgot it, or mentally blocked it out after being marooned on Jakku. And well, Luke was a mechanic, had no problems picking up a blaster or the Falcon's turrets, and while not pulling mind tricks (keep in mind it took Rey a couple tries to get it to work) he was able to use the Force against the remote while blinded. About the only thing he DIDN'T have was the languages, and that can be explained by Jakku having a much more varied population than most of Tattooine.
I never really felt any sense of foreboding with the young sith lord as I did with Vader. Vader's menace was his inhuman detachment and cold self-control when dealing with failure - once the new guy started throwing temper tantrums and removing his mask, all of that was lost. Just no real suspense there. Probably inevitable we were going to learn he was Han and Leia's son, but I figured it would've been when they met at bridge towards the end. Yeah, so the son felt Han land on the planet but didn't feel him 20 feet behind him on the bridge? He turned out to be a very rookie sith.
Kylo Ren is NOT a Sith, he's a Knight of Ren. Yes, there is a difference. And the entire POINT of Kylo is that he's trying to live up to who he thinks Vader is, but has doubts and uncertainty about himself, and is still struggling with the Light vs. the Dark. Rey even points it out when he tries to mind rape her that he's trying to BE Vader but is NOT Vader, and the thought he can't live up to that is scaring him. Thankfully, I'm glad they DID make him much more vulnerable. I'm also glad they didn't drag out the reveal.