I did not enjoy Below Zero to the same degree as the original Subnautica, which sucks because there are many new mechanics in this game that I really like or at least think are neat ([spoiler] Cold System, Quantum Chest, Recycler, Vehicle Teleportation, Monke tithe collection [/spoiler]), but, ironically, lack depth. The underwater world feels smaller with not much reason to go to more than 2-3 biomes for materials or base building, new mechanics are situational/heavily localized, and just another way to get resources if picking them off the ground felt too same-y for your gameplay. There's no real way of conveniently moving a lot of resources easily. There are ways of attempting to do so with new mechanics, but they are, at best, only doable in the late-end game, tedious, and the hard cap is still nothing compared to having a Cyclops.
Leviathan scaling and encounters are basically the same as Subnautica 1, but creatures here have less "energy" and are more predictable. Mid-late game, there's no consequences for letting a leviathan tickle your vehicle, which depending on who you are is the best thing, or something that removes the fear factor and appeal from a Subnautica game. Most hostile leviathan designs feel like they are trying too hard to be scary/creepy by being overdetailed, and don't really have a characteristic identity beyond "fish" (some do, though).
Perhaps I'm not the target demographic, but the story didn't grip me, it's not the worst, but the characters are more caricatures of "society" than real people with motives beyond "I do/don't want to be part of a corrupt system." While obviously they have more personality than the zero-personalities of Subnauctica 1's "characters," their one-dimensionality makes it all more fatiguing; you don't have to engage with the story, in the same way you can just eat around the flies in your soup. The player protagonist doesn't have much personality (which could have been fine for a sandbox game) except for "having a sister," egotistical, and holier-than-thou (while mowing down her 14th Pengling family and exploiting a wilderness for its resources and wildlife for lavish vanity). I would have been personally been fine with characters being one-dimensional, but the story has an air to it that expects you to take it seriously.
"If you didn't like it, why give it a positive review?" There are things I don't like, yes, but, it's a Subnautica game. Below Zero could be thought of as a new save DLC more than a standalone game for people who just want more Subnautica. I have a feeling that most people looking to play this will have already played Subnautica 1, So if they're considering getting this, then yes, there's enough here to justify the playthrough, and the non-human parts of the story are interesting enough to warrant engaging with it, just understand that it's a "good fast food burger:" not the best, but it can give you a fill if you're looking for something different.
