Disclaimer: I only watched the first series, and I know most people actually like the show. Subjective dislike abounds here: you can prefix "In my eyes only" to every sentence below. I speak about the show, not the books here.
So first thing that struck me is that I'm not emotionally invested in any character. I'm not rooting for anyone. I don't care about them, and they don't have any drives or characteristics that immerses me. They are wooden, shallow, unrealistic caricatures of banal archetypes.
Amos is probably the most obvious example: he is a grab bag of overly obvious machismo and pulp tough-guy slapped together in such a way that he is concurrently completely unremarkable, unrelatable, and unbelievable. Every time he got on screen I would lose immersion and start wondering how the writers could settle for such cheap tactics and obvious low effort characterization - they basically bludgeon us with a few standard examples of machismo and violence. I end up completely uncaring about the character and I don't identify with any of his supposed drives. It just screams artifice.
Another example: the same obviousness and wooden characterization goes for Avasarala, but it's compounded by properly bad acting. I don't know what happened here - Aghdashloo was heart-achingly convincing in _House of Sand and Fog_, and her voice is so beautiful that one would think she'd be able to carry any role. But no - Avasarala is a character so badly scripted that I go from watching the story to irritation in 2 seconds flat.
In a variety of ways this goes for all characters. One cannot get a grip on their internal worlds and start caring about them - what should've been the complexity and contradiction inherent in human behaviour comes across as accidentally acting in a way counter to their shopworn character traits. Why does Miller care about Julie? Instead of adding meat (his exception to his internal rule) to Miller's character it just feels inconsistent with the rest of his actions. They flap about in thinly disguised cliché suits - thereby often acting unexpectedly in completely uncharacteristic ways (think Naomi for example), so they just come across as unbelievable.
The world building is also so-so. For instance, I found the Belter patois to be more irritating than supportive, mainly because it comes and goes in various amounts without enough consistency. The whole backstory is also a bit... bland? Made up of cheap tropes? Two untrustworthy governments with tension between them, a possible but oh so avoidable war, oh wow that one is a double agent all along, a group of disadvantaged blue-collar outsiders being manipulated by everyone, meh. The politics is so cartoonish and predictable as to break immersion, again. Oh, and how many hackneyed sci-fi inanities do they want to repeat? Hypoxia much?
The acting: least said, soonest mended.
The random violence. I don't care about the characters, and I don't worry if they live or die, and the violence is still so unexpected and over the top that I still roll my eyes. Also, Amos again. How do characters go from trying actively to kill each other to backing each other up in a fight in 5 minutes?
The CG is good, though.