I've read a couple of Film Crit Hulk's arguments, and I have to agree, his shtick is annoying and it makes him a chore to read. Which I think tends to make his arguments go in one ear and out the other, which dilutes my ability to agree or disagree with them, because if I'm not putting 120% effort into reading him, it's as if I don't give a shit.
In this instance, I think reading him wasn't worth the effort. I don't think JJ is a perfect director, and I'm fine with people criticizing him/TFA, but I don't (currently) think his shortcomings are that big of a deal. At worst I think he's "not that bad".
I disagree with FCH on the oldschool Star Wars fanservice getting in the way of the movie. I think it was vital for JJ to draw a line connecting the Original Trilogy to the new Sequel Trilogy, in order to flush the Prequel Trilogy and bring the franchise back around again. You can't just make a great Star Wars movie that stands on it's own (if such a thing was even possible for JJ), because of the context of the franchise, it's history, and it's new owners. Fanservice was the right call.
Finn, and really any of the new characters, have no inherently distinguishing characteristics or qualities though. If you tell me that Finn is supposed to "brave", that really isn't a quality that I immediately associate with him. Because by that measure, basically everyone is "brave".
I think Finn has some immediate character shorthand built into him, because he's a Stormtrooper. We've seen them everywhere and now we're looking at a humanized one.
When Finn went through that first battle with a bloody handprint on his face, and refused to shoot anyone (prompting his CO to assume his gun was broken, potentially getting him in deep trouble for dereliction of duty), I think it brought home the gritty reality of a Stormtrooper. I was reminded of something I've heard about drafted rookie soldiers, and how on their first day they always miss their kills on purpose.
One definition I like is that courage is the ability to do the right thing in the face of fear. If someone's not afraid, that's fearlessness, not courage. You need fear in order to have courage. Finn was clearly terrified, hyperventilating when he took off his helmet, but he put one foot in front of another and moved forward and rescued Poe Dameron so they could both get away together. Finn pretended to be a hero, but with his job done, he wanted to run away, and he wanted Rey to run with him. Rey wouldn't run, so Finn left her and ran. But then when she got caught, he overcame his fear and went after her. He faced down a Sith Lord and took a beating, which bought Rey time to get back up. Finn's great.
I don't believe Rey is a Mary Sue, but I think she's overpowered. Some of it was perhaps-misplaced female empowerment (not needing to be rescued by Finn), while some things like beating Kylo Ren's Force powers (twice) and Jedi-mind-tricking the guards felt unearned. Some things about her work though, like how effortless it was for her to steal the Falcon and escape. Finn was trapped with the Stormtroopers because he didn't have the means to escape. Rey had all the means in the world to escape (by way of working salvage, making her familiar with the pile of junk which is the Falcon), but she was
mentally trapped working in a shithole, because she was left there by her parents, and she's still just a little girl waiting there for them to come back.