Now that I'm farther in, I can say that Subnautica is an exceptional game. It has a knack for opening up new stuff to see right as you think you've found most of what there is at a given tech level.
"Okay so I feel like I'm starting to understand the scope of things -- oh wait, heck, there's two new biomes!"
The game is made much stronger by using a fixed map instead of being purely random/procedural. It helps the pacing a whole lot.
Also, completely unexpected from an exploration/survival game, Subnautica has an great sense of humor.
The big challenge is that you have to be self-motivated as a player. There's not much pointing you towards you goals or even really telling you what they are.
@starkatt It was interesting to learn that, in an abstract pared-down game, when I'm bad at that sort of task compartmentalization in the orthocosm.