I used to be a huge Star Wars expanded universe nut up until it effectively died for me somewhere around 2012. They decided to ignore all that and start the new film with fresh continuity. That annoyed me enough to kill all the hype and I went to see it with pretty fairly attitude, with some of "it better be good to be worth it". I came out from seeing it reasonably entertained, thinking that it's not great like episodes 4 and 5, but not bad like episodes 1 and 2 either. Fairly fun with some flaws. Good watch if you can turn your brain off. I remember thinking that I'd have to wait for later for my final verdict because I had similar initial reaction for the new Star Trek, but it became more "meh" for me in the following months. Seems like similar thing is happening with this one.
TFA is like a pretty box with nothing inside. Seems appealing at the surface, but it really gives you nothing. The most substantial thing this movie gave me was new appeciation of Lucas' work, yes even the prequels. He might have failed with them, but at least he was trying. This isn't trying but just rehashing old stuff, like the media seems to love doing in these days. Same old Star Wars, same old plot, same old ships, same old locations (albeit with different names), same old Han Solo. The new characters are nicely acted, but too poorly conceived and developed to make us care about them. Well, Poe Dameron might have been cool with more screen time, but they'd more likely ruined him too somehow.
Right, those were my main points. I'll comment OP's points with more detail now, as I feel entitled to do as a former Star Wars geek. Feel free to ignore
- Yes, we are given little information about the Republic, First Order and Resistance and have to deduce the rest. That doesn't bother me because I'm keen to guess, but they will probably go into more detail in sequels, and I will be slightly upset when my beloved theories turn out to be wrong. For the record this is my deduction: The Republic and First Order were in a cold war situation mainly because the Republic was too cowardly/lacked political will to attack and First Order was too weak to do so, at least until they built their superweapon. The parties within the Republic that wanted to do something about the Order formed the Resistance to wage a proxy war for them. The Republic officially denies everything, but supports the Resistance.
- Yes, Rey is a Mary Sue. This is too bad because I was initially inclined to like her. When her Mary Sue abilities started to pile up, I simply couldn't. She's good at everything. When she tries something she's never done before she inconsequentially fails once and then does it perfectly every time. All the good guys seem to like her right away. She never has to learn their trust or anything.
- Kylo Ren is actually pretty interesting character if you consider the whole Han, Leia and their messed up son aspect. However, it was completely idiotic to make him the primary antagonist. I liked him because he's basically Jacen Solo/Darth Caedus from the old expanded universe, albeit whinier and more insecure. Rey beat him so handily that it will take a lot of effort to build him up as a credible threat again. Might be better to just scrap him for some new bad guy.
- Yes to pointless fan service. As I was watching it I thought they were so busy paying homage that they didn't have room for a movie of their own.
- Yes, the Starkiller Base isn't the stupidest superweapon I have ever seen, but it's still pretty high up there mainly because of bad case of "sci-fi writers have no sense of scale". I don't expect realism in a Star Wars movie at all, but this killed my suspension of disbelief a few times. What is worse, it seems to be there just because the writers think there has to be a Death Star. It doesn't have the impact the superweapon that destroys entire star systems should have. Sure it destroys the Republic capital, but no one cares. We get a shot of Han looking at the sky (at completely different star system, mind you) and say "It's the Republic!" but that's it. Then it threatens the Resistance base with little tension and gets destroyed by Poe almost off-screen.