The story did feel small, but that was on purpose. Re-watch episode 4 and you really get the same feeling. Sure, the Deathstar seems like a huge threat on paper, but think about it. Leia didn't even seem all that upset about watching her home planet get blown up. She just sorta sighs. No sobbing, no screaming, just a "well that fucking sucks". It looked like she found out her goldfish died rather than watching billions of people and everyone she ever knew or loved get vaporized. And even when they fight the Deathstar they don't send much of a fleet. Just like this movie the attack consists of a few X-Wings and that's about it. It wasn't until episode 6 that we got a truly grand feeling Deathstar battle.
Did you honestly care when Alderaan blew up? I can honestly say I've never met anyone who did. Sure, Leia was from there, but we as the audience had no real connection to it. It was just there. And like I said in my last paragraph Leia didn't exactly sell it. Hell, she just sorta forgot about it almost immediately and it's never brought up again.
Kylo was too nerdy in my honest opinion. The temper tantrums were laughable. Like seriously it felt like a SNL sketch spoofing Star Wars sometimes when he did that shit. I mean I laughed, but you shouldn't be laughing at the guy who's supposed to be your big villain in a movie like this. It was like watching an 80's cartoon most of the time. I will say though that it's not a problem if they're going somewhere with it. If he evolves from this over time thanks to his training with Snoke then I'll be totally behind it. The idea of a whiny little bitch turning into a badass machine of death is actually a good one, and is what Darth Vader was supposed to be. However, the prequels fucked all that up and Anakin never got the big payoff that should have come from it. Hopefully that's what they're doing with Kylo and it pays off.
The Han death scene I'll give you, but I don't think it was bad. Predictable =/= bad automatically. I feel like they did a good enough job of making it interesting, and I will give them credit for legitimately making me HATE Kylo Ren with a passion. Like I legit cheered out loud when Chewie shot the little bastard. I'll definitely be happy to see him die at the end of all this which is something I can't say I've felt for a bad guy in a movie in a long time. Good on them.
Finn's emotion (or lack thereof) over being a Stormtrooper is kinda understandable. 1) Star Wars isn't exactly known for showing lots of emotion in the first place (see Leia and Alderaan). 2) He was a Stormtrooper for literally 1 day. The dude was seeing his first combat ever and saw what was probably his only friend in the world die in front of him. It's totally understandable that he'd turncoat on the First Order when he never had any real connection to them in the first place. I'm honestly surprised more Stormtroopers didn't/don't turn.
Rey is mysterious for a reason. We're almost certainly learning more about her in the sequel movie. This wasn't meant to tell you everything. This is the first chapter in a series, not a stand alone movie. If they gave everything away now they'd have nothing to give us later.
I'm assuming you mean what happened to Poe after they crashed on Jakku, yes? Well, he tells us. He woke up in the desert after being ejected just like Finn was. Probably found a radio or something and had the Resistance send him a ship to pick him up. Pretty simple. If you mean where did he come from in the first place, well, why does that matter? We never find out much about Han Solo outside "he was a smuggler and pissed off Jaba" and finding even that much out took 3 movies. Like with Rey I say just give it time. They'll probably explain more about him later in another movie. Remember we're getting at least 2 more main series movies and a bunch of spin-off movies too. Hell, they could do an entire movie on each of the three main characters if they wanted, and they probably will.
As for Starkiller base and the comparisons to Episode 4, well, yeah that's exactly how it was. You're either gonna hate that or love it. I feel like they did just enough to make things feel fresh while also keeping a lot of familiar elements to give us that nostalgia feeling. And don't forget that George Lucas once said in an interview that he intended for the trilogies to "mirror" each other. I feel like Abrams was just trying to live up to that but in a good way instead of the shit way Lucas did with the prequels.