Quill and Gamora talk a few times about their (non)-relationship. I think this is adequate, and I'm glad it was touched on. Peter eventually has to accept this isn't the Gamora who loved him and vice versa, she's like a different person. By the end of this film, they've finally shared some things together, and you see Gamora at least coming around to Quill not being a loser and even having some admirable traits she doesn't have -- i.e., she gets a glimmer of why her other self MIGHT have eventually loved him -- but that's not who she is. She is not the Gamora who joined Quill to use an Infinity stone, or who bonded with Nebula as a sister during their catharsis on Ego's planet [in Nebula's favor, she already can acknowledge this -- they are in regular contact and see each other as friends, but the depth of emotion is not there]. This Gamora wasn't put through the things that forced the older Gamora to change; this Gamora thought her father was a psychopath and she wanted to stop him from destroying half the universe, but otherwise she's still a ruffian and pretty rough around the edges. Saldana has a lot of fun with it.
Nebula is my bae and she has one of the best arcs in the whole series. She starts as a one-note villain, then you find out she's a victim and ultimately really wanted her sister's love, which was driving her fury at her. in Infinity War, you see the bond they both have with each other (when Gamora risks herself to tell Thanos about the Soul stone because he's torturing Nebula and she can't bear it -- and Gamora walks over and tenderly touches her distorted suffering sister's face). In Endgame, you see her still awkward but tenderly doing small things to care from Stark, later holding Rocket's hand quietly -- and then later she tries to appeal to her younger self and Gamora, to stop Thanos. Here, the film opens with her even more strongly caring for Quill without judgment, although she's discouraged by his hitting rock-bottom from Gamora being alive but not interested in him -- they took time to develop her, and when she is no longer a bitter victim of Thanos' cruelty and can have space to move past it, despite being part machine, she shows a deep capacity to nurture. The end of her arc is not a surprise. I love her so much.
Drax also has an interesting arc and it makes sense, you just don't notice it building. Remember his thoughts on dancing?
Mantis' arc is understandable but feels like it was just tossed in -- no issues with it, just... okay.
Same with Peter, but I get the feeling where he ends up is a temporary thing with him. Still, what he does is a pretty sweet twist and a great way to come full circle.
Groot doesn't really have arcs, but he's pretty amazing. I did feel like his talking at the end was cheap fan service, but whatever. it SHOULD have been something inane or witty, not sappy.
Rocket, also, I felt like he didn't necessarily have an arc (except to realize it was okay to trust and have friends again?) But this film is mostly about understanding what has been holding him back. So sure, allowing himself to be loved basically is his arc.
I wish the humor had been more throughout and more consistent so it was all blended better. Instead we get obvious jokes that aren't often funny, or are just somewhat amusing pop culture references (like when Groot's head pops legs and scuttles away a la The Thing).
There is a big problem with the film essentially sidelining Rocket for 75% of the film in terms of separating him from the other Guardians. A lot of the best interplay happened between Rocket and others -- and that isn't happening here, so it weakens the film some. The plot does keep Rocket in the film by making his memories and backstory -- his "life flashing before his eyes" so to speak -- central and ongoing, we know it's all flashbacks, but they happen in order like another subplot. It's so awful and evocative, and it helps one understand exactly what Rocket is and his capabilities. The High Evolutionary finally made a creature (among zillions) whose creativity and intelligence transcends his own and yet he can't even replicate it -- and since he views himself as a god, he can't live with that... nor can he accept that a disgusting animal with so many flaws (a very young Rocket) was capable of solving a problem that he could not and basically showed him up. See it from Rocket's view now: What would you be like if you knew that you were created by God, yet God also actively despised you when he isn't just viewing you as something disposable and unimportant? Rocket also chooses his own name, he and his friends all do so as to not be known as numbers; it's touching to understand why he calls himself Rocket. I'm almost crying typing this, it's so damn moving.
So this makes the heroic end of the film even more touching, to know what the Guardians have done. Maybe they've saved the galaxy twice (by recovering the Power stone, and then by killing Ego), but their final act here is so personal that it seems to dwarf those largely impersonal acts in some way.
I do have to say: I really HATED the thoughtless "Oh Quill almost dies frozen in space" thing at the end, it's not climactic, and it's shown up in both other films so it felt super-uninspired. If you wanted Warlock to save Quill, he could have been in trouble in many ways other ways, including wounded and trapped back on the HE's ship somehow, and Warlock would have still been able to save him. We don't NEED to see Quill now almost freeze to death in all of its grossness -- Gamora almost froze, then Quill saved her and almost froze (in film 1), then half the ravagers die frozen and then Yondu in the climax of the 2nd film... and now again? WTF?
[It actually made a lot of sense in the second film for Yondu to die that way -- he got half his crew (the loyal ones) killed, and watched them all die. So this is another reason he was okay with dying that way, to save Quill; he was already living on borrowed time, and all the men who were true to him had died in that way, and so as their captain it was a kind of closure to die in the same manner as they did, while saving his boy.]