I agree there was a lot of fan service but I enjoyed all that, and I don't think it will have got in the way for non-fans watching it. The whole giant death star thing, I'm not sure, I can't remember if they fully explained the intricacies of it (I need to watch it again). They had to tread a line I suppose of not bogging things down too much with explanations.
I really loved Kylo Ren to be honest. He was outstanding to me. He seemed really disturbed and the fact that he had this emotional instability made it all the more interesting for me because with someone like that you really don't know what they're going to do next. Rather than him being straight-forward 'evil' if you know what I mean. He
is evil but his intentions (and emotions) aren't fully clear which gives him an unpredictability I found really interesting. I liked that he wasn't a stereotypical sith type person. I'm not even gonna start on you comparing him to Annie haha!!!
I think there will definitely be more lightsaber fights in the next one. I hope so anyway. It would have been tricky for them to write more into this one, as you say you need force sensitive characters, and Rey was only just starting to discover hers. I kind of feel like as much as it would have been great to see more lightsaber battles, they probably would have felt out of place in this story, based on where these characters currently are. And although the Rey/Ren fight was brief and a little messy (because of her lack of training) It needed to be really. I thought that it fitted perfectly and there were small moments of greatness rather than it being a grand battle. Also, they probably don't want to throw everything into this one movie and go overboard, save some for the next ones.
It's a good point about Finn and the lightsaber. I just assumed that his storm trooper training might have helped with him fighting, along with Ren being injured. But him being force sensitive would make much more sense, along with your theory about that being the reason why he couldn't go through with the killing at the start.
I'm gonna have to think more about it all when I've watched it again lol!!