In many ways, this is a straight-forward, old-fashioned space adventure. A deep space vessel - in this case, a tunneling ship, used to cut stable wormholes for interstellar travel - takes a job that turns out to be less straightforward than anticipated, and we see how the crew deal with that, and how it affects their relationships. (And I'm not trying to belittle the story or the genre; I know any story, when broken down into its basic elements, seems simplistic).
The difference here from the vast majority of these sorts of stories is the focus. While they usually put the adventure itself in the spotlight and character tension rises from that, Becky Chambers concentrate on the characters and their relationships from the start. We begin by being introduced with a new crew member - Rosemary, who seems very much a surrogate for both the reader and Chambers herself - although the third-person viewpoint quickly diversifies among the multi-species crew so we see events from different perspectives. There is a great deal of introduction to this universe, the GC (Galactic Community? Galactic Council? Something like that) to which humanity is a a fairly recent entrant after leaving a polluted Earth to repair itself and splitting into two factions - those settled on Mars and the Exodans, who live almost entirely in their ships and space stations.
The relationship-focus and the grimy, real-feeling level of the tech - along with a sharpness of wit and dialogue in the writing - are certainly why this book has attracted so many comparisons to Joss Whedon's Firefly although, if anything, here the story takes even more of a back seat to the personalities. The events are largely low-key (about which I am not complaining as I am thoroughly sick of save-the-universe stories) and the 'small, angry planet' of the title, and the main mission, does not even figure until the last fifth of the book - although, I guess, it is a long way there. It is, however, these relationships and the characters that carry the story and set it apart. The characters are fairly well realised and it is these relationships - both with each other, and outside of the family of the crew - that give us the bulk of the interest and peril. And they are very much a family, something to which focus is brought by the mention of the family structure of the reptilian Aandrisk, of whom Sissix, the ship's pilot, is a member. They are born into and raised by a hatch family (of probably unrelated individuals to their parents), choose a feather family - or, rather, a succession of feather families - of like-minded individuals throughout their adult lives, before settling into a more (but not entirely) stable house family to raise young given into their care when they are older. As well as an intriguing social structure that holds echoes of many ancient human tribes, it is a clear metaphor for the friendship groups we build and which are - often and for many people - more important than the families into which we are born.
Along with the very 'liberal' political (both small 'L' and small 'P') - there is no distinction made between binary or same-sex relationships, indeed interspecies relationships are touched upon, and gender issues are foregrounded - I imagine this is the sort of SF that makes the Sad Puppies rabid. So, kudos to chambers for that.
However, this is 'just' about gender politics (in fact, i'd say it isn't about gender politics at all, but I guess, unfortunately, taking the position that who you love is no big thing is a political stand). The theme of the book is about identity - those relationship identities, yes, but also about what it is to be a person. As well as the personhood of each of the alien races, the status of AIs is one of the big questions that this society has yet to address.
Even if you are perfectly happy with the type of SF that this - which I absolutely am - there are problems with the book itself, on its own terms. The crew of the Wayfarer are a bit too nice to each other - with the exception of Corbin, the stand-offish, persnickety algae tech (one of the power sources for the ship) who nobody else really likes, they all get on far too well with none of the annoyances that you would expect of people living in close confines for long stretches of time. I would have liked to see more interpersonal tension, if only in small ways.
However, Chambers does generally write the characters and relationships well, but this can't be said for some of the physical threats. There is a big action scene near the end that you can almost miss if you're not paying attention - yes, partly there is confusion as it is from the view of the crew who aren't expecting it, but even so the whole thing needed rewriting. But, hey, it's her first novel.
There is a far more important flaw, though. In showing us that all the five species who make up the crew are people, with the possible exception of the wonderful Dr Chef, we lose any sense of their alien-ness, except when it's explicitly stated as with the Aandrisk family structure. Okay, the fact of their personhood, that "we're all the same under the skin", is kind of the point but, for all the diversity in the liberal outlook, this has a homogenising effect that slightly undermines the message. The only species that did seem at all alien were those that were threatening - the Toremi, the civilisation to whom the Wayfarer is travelling, and the militaristic Quelin, who seem to have it humans generally (and who I pictured as the being like the Vogon guard tasked with throwing Arthur and Ford out of the airlock in the TV version of Hitch Hikers' Guide to the Galaxy...) This does seem to leave us with the lazy idea that explicable and similar means friendly whilst different and confusing means an enemy, which I doubt very much is what Chambers intended and, I'm sure, something she'll address in the further volumes. And, again, it's a very hard thing to write, but here it is very, very important.
Oh, and just one more thing. This isn't a complaint about the book at all, but about many of the reviews. Stop calling it space opera, it isn't. Space opera is the grand guignol of the spaceways - it is big storylines, overblown, huge events painted on a massive canvas in broad strokes. Often with fat ladies singing. It is Peter Hamilton and Alastair Reynolds and the sorely missed Iain M. Banks.