I mean, first of all, I guess obviously: not spoiler-free.
Okay. The most obvious thing:
Not only is this just an absolutely stellar set of tunes, the absolute punch-in-the-gut of feelings that he treasures it so much because it's from his dead mother just made me feel so much more emotionally attached to Peter and the mother that we only met onscreen for about 45 seconds.
Also, it was pretty easy to make these character designs into tacky-looking troll dolls (I was particularly worried about Thanos), but as usual, on the design front, Marvel hits movie adaptation out of the park. And I will concede that bigger fans than me are probably unhappy with some of the bigger inaccuracies (Knowhere, for example, is not where the Collector's base is). I think they've found a sweet spot for old and new fans both.
More than anything, the characters are perfect. Guardians of the Galaxy is essentially an exciting version of Friendship is Magic (I assume? I've never watched Friendship is Magic, maybe there are explosions).
If you've got even a little bit of estrogen in you (males, I'm lookin' at you too), you cannot deny at least a little bit of an attraction to Peter Quill. Not just because he was already adorable way back in his parks-and-rec days, but now he's got the whole charismatic Star-lord thing going on. And yes, he's an ass, but he's got heart the whole way through. His character development isn't some obnoxious 180 switch; you can see him ride it out. And I love that, because more than anything else in the movie, I feel like they made his heart so real.
Rocket and Drax are absolutely hilarious. Drax's inability to take things figuratively is golden for any deadpan-humour enthusiast.
But then... you get a little bit of insight into them. Drax's story we get right off the bat, yes. Not Rocket; he eventually gets tense enough to reveal how much he hates being called a rodent. Which might sound like a problem that's hard to identify with and easy to laugh off, but it's not. He hates being belittled for being what he is, and there's a lesson there. Later on in the movie, someone calls him 'vermin' again, and he doesn't flinch. The audience doesn't get a "knowing look" because he gets it all the time. He hates it, but it's always been like this.
And you carry on in the movie, just feeling so attached, while they're already off cracking jokes or shooting bad guys again.
She's not flouncy and flirty. She's got a nice build, but still realistic body proportions and clothing; in the comics, she might as well be wearing nothing but a ribbon, but Marvel didn't take this as an opportunity to sneak fanservice in under the guise of keeping the movie canon. She takes charge of her own life. She's doesn't reveal her endearing, emotionally vulerable, sweet-as-bubblegum personality, because that's not her. And when he tries to get her to dance for the second time, Gamora the BAMF stands hard on her firm 'no'. I love that. She's exactly the kind of heroine I want to be.
And then everyone's favourite. The second I saw a swarm of t-shirts circulating around with "We Are Groot", I knew that Groot was going to die and that those would be his last words. It seems more and more inevitable throughout the movie, considering he's by far the most loveable anything I have ever seen, so I'm glad that I was mentally prepared to say goodbye. It gave me the opportunity to really fall in love with this giant, murderous tree-puppy, and then accept that we had to say goodbye to him (yes, I cried). The amount of satisfaction and absolute glee that came when he came back as baby Groot with a tiny face made everything okay in the world.
Ugh, and I love the absolutely cheesy way they hold hands to handle the power of the infinity gem.
Plus, the man, the legend himself's cameo is so perfect for the sole reason that he's called a pervert.