A few quick points here and there regarding most comments after my last one.
Akalaabeth's deus ex machina is not a correct example. The desperate situation was to destroy the Starkiller base and the map that allowed Rey to find Luke had nothing to do with that. It was structured as an epilogue, not as a deus ex machina. Insufficiently explained why R2 would turn on at that point? Perhaps. Still more than 14 parsecs away from any reasonable interpretation of what "deus ex machina" means. The crawling paragraphs were on how Leia was trying to find Luke and little BB droid had the last parts of the map to do so. My guesstimate is that R2 was "off" until Starkiller was destroyed in order to smooth out the script. First, destroy Starkiller, then find Luke. Easier on the audience.
I agree with Akalabeth that the point when Finn went bananas was when the blood of his comrade went into his helmet. This is known to happen to humans, it felt real to me. And then he witnessed his teammates massacring a whole unarmed village. I interpreted it as a "Lacanian moment" of truth slapping on his face, and he went straight into panic mode, eventually decided he couldn't do this anymore, and so on and so on. We could endlessly argue if this is good or terrible writing, I feel that Boyega delivered it so well that I was sold on it.
Regarding why should we care about those planets and so on, I also agree this is the part that was lacking. And it wouldn't take that long. I could easily picture one or two extra minutes in the movie right before these planets were being destroyed where they would show a bit of life, of military analysis within these planets concerning the danger of StarKiller, a few scenes spanning a huge Republic fleet hovering over the planet, and then we see the beams destroying everything with these generals or politicians amazed that this thing was already "fully operational". I don't know how tight that idea would be, but it would also explain why the Resistance had to plan an attack on StarKiller base with just a few X-Wings, with a one-liner like "This is all we got within distance now".
Oh come on. This is a massively dishonest answer to my questions. You answered the preamble (not particularly well either. Poe could have simply destroyed the map if keeping Luke safe was what was of importance.) and ignored the meat of the question.
Keeping Luke safe wasn't the plan, finding him was.
But why would anyone have faith in Luke to put the universe right when he'd already walked away from the whole thing at one point? Why would you trust him? Why would you expect him to come back? Sure Han and Chewie know him, but everyone else seems to have a lot of faith in him for absolutely no reason.
I didn't see it that way at all. Leia wants to find Luke, Han also seemed to think that was perhaps a good idea. Finn and Rey are just trying to survive in this mess they find themselves in. Eventually, two things happen. It becomes abhorrently clear that this StarKiller base is something they should destroy, and Rey is "called" by Luke's lightsaber. She's drawn to the force and by Luke's lightsaber. That seems enough motive for someone to track him down and ask him some really hard questions.
Regarding "the second coming of Christ", well we could have a good laugh here because given how he's the son of someone who didn't even had a father that's probably more literally true than what you wished for

, but more importantly, in ESB the emperor tells Vader that the "son of Anakin" was the most dangerous thing out there against the empire. So it's quite canon this idea that a single Jedi could be really bad for any wannabe empire to take hold of the galaxy.
More importantly, why are you so accepting of Luke doing such an unlike Luke thing as going away and leaving the universe in peril?
It has been correctly replied before. He's depressed that he utterly failed to recreate a Jedi academy, being betrayed by Han's son. He perhaps even thought that the world would be now a better place without all these Jedis and Siths going at each other. I think we'll know more in the second movie, but I didn't find it weird. Yes, it was a bad judgement on Luke's part, which only makes him human. Which is not bad writing.
If you're going to say that the characterisation of the films is really good, it's not nitpicking to point out example of how it wasn't. It not nitpicking to show real example of how the characters don't seem to be acting like real people would. You're basically taking the point of view that you liked the film and therefore everyone else should or they are nitpicking. In general I found the characters in TFA to be reasonably good, but if people are going to start elevating them over those in ANH or claiming that it was perfect, I'm going to take issue with that, there is a lot they did wrong.
Nah, I found many flaws as well, but I also found many weird and irrelevant nitpickings in here. I think the movie was generally good, and I had a blast of an experience because it was the first time I took my eldest to a non-animated movie cinema, and it was IMAX 3D and so on. My expectations were between low and reasonable, given JJ Abrams. And they were surpassed, becuase I really enjoyed the characters and Rey especially. It definitely won't be the best movie I'll see in 2016, nor was that the point. It was, however, a blast of a spectacle.