Honestly I actually liked BZ more than the original, but it's difficult for me to figure out why exactly. I play those games a bit differently from the proper way, I basically ignore the plot beyond what is needed to get the tools and location I want and mostly just play them to build and decorate bases in a pretty environment with survival elements to make the base feel like it has design requirements and resource constraints, like Minecraft almost, but I'm not going to say that BZ has prettier environments for that either. Might be the creature design, my favorite subnautica critters are mostly in BZ, though not all, and they feel like they fit together more tightly than the ones in the first game, if that makes sense, like one can look at certain features and get a greater sense of them all seeming to descend from a few common ancestors.

You had to discover the world yourself in Subnautica, they pointed you to an island one time, the rest was reading PDAs. You were a silent protagonist, so it was easier to immerse yourself in the world.

Below Zero was so disappointing in every regard except more base building options. (I cant remember if they added them to the original or not in an update). Why did they make the protagonist speak??? Ugh.

You go down on the planet to find out what happened to your sister and you get pretty much railroaded (waypoint here, waypoint there) into leaving without ever even acknowledging your sister again, unless you remember and go looking. All the enemies in BZ had the same "munch munch munch" making them all so uninteresting. The world was small and empty, it literally had no depth. The entire land portion was such a dumb addition and the worm was just incredibly annoying rather than something daunting to deal with.

Subnautica is one of them games you wished you could play blind again. Below Zero though...

It's one of them too often occurring cases where developers don't know what made their original game good.

You absolutely nailed it.

KrappedOnAI