My two biggest beefs with the narrative involve the new Jar Jar Binks - the character of Finn and the inclusion of the characters from the first trilogy, who are now obviously older. The two problems could actually fix each other if written properly.
I want to speak first about the original characters. Recall in A New Hope, Alec Guinness played an old Obi-Wan. He was an old man who did mostly talking and only acted when necessary. Here we have Han Solo - a 70 year-old man, running around like he is in his 30s. That's just.....weird. He should be sitting on some star system managing smugglers, not doing all the grunt work. Watching Ford wobbling around was awkward to see. And then to see Organa in field gear at her age, rather than being a princess somewhere in a counsel or doing managerial stuff - see old people in real life aren't running around doing anything. They mostly sit, talk, and manage things.
Now the Finn problem. This character has serious issues. He is mostly comedy relief in a main character, which is awkward, but Lethal Weapon did it with Danny Glover, so given the right circumstances, it can be done. The problem is you have someone who in terms of personality is both retarded (mentally underdeveloped) and a child (emotionally underdeveloped). Now, I could buy that if the other stormtroopers acted that way, but this is the first I've seen of a stormtrooper of this kind. They all seem rather competent and can follow orders pretty expertly even if they aren't as skilled as the main characters. Finn acts like someone who has an IQ barely above a Forrest Gump. He is a cartoon character given life. The fact that he wield a lightsaber like any other weapon and as well as the force-users is a problem as well.
Now the main narrative problem is that you need Solo and Organa in the thick of things - because of the central conceit. I understand that, but here is how you could accomplish that - Make Finn Solo's apprentice and surrogate son, someone who took over the smuggling business after the falling out with Kylo Ren. This creates natural tension between Ren and Finn and ties Finn into the mythology rather than just being some joke character that was in the right place at the right time. Finn's goofiness could be explained by being Solo's apprentice and Finn being the only person dumb enough to take orders from Solo - Finn is loyal to the old man. What a terribly wasted opportunity.