Pro Incredibly satisfying tool-building mechanics
The whole process of creating new tools is mostly done using a sophisticated 3D-printer available from the start of the game. You gather various resources and transform them to create tools that you will need for your survival.
For example, organic matter gets printed into raw carbon, combine carbon with some zinc and you get a battery; combine that battery with some glass and you get a flashlight that helps you see in the dark. It's very straightforward but incredibly satisfying when you build your tools, especially since you are doing all of this while swimming in an ocean filled with predators.
Pro Has a mode for people who just want to explore
Subnautica has four game modes: survival, freedom, hardcore, and creative. The first one pits the player against the elements and forces them to think about food and water through its thirst and hunger mechanics.
The second game mode basically removes every mechanic that is related to surviving, such as the aforementioned hunger and thirst mechanics. It also leaves all the items in your inventory after you die. In other words, it's a game mode for people who want to explore without having to bother with surviving. Which is nice, since Subnautica has a lot of stuff to explore.
The third one, as the name tells us, is more difficult. Not only do you have to worry about health, oxygen, water and hunger, you will have to worry about your life. Since this game mode gives you one chance to live, no respawning. If you die, you'll lose your progress and will have to start over again.
The fourth one will let you build a base of your dreams without worrying about food, oxygen, or thirst. It is the best mode for people who want to build and don't want to gather resources.
Pro The sound effects really set up the overall atmosphere
The sound design in Subnautica is amazing. Even though the soundtrack can be bland at times, the sound effects for the ocean noises are incredibly atmospheric and really help to get the player in the mood for exploring a vast, alien ocean.
Pro Great theme and looks
The game takes place underwater, and with that it looks quite good for an underwater game. Looking up at the sun through the ocean looks pleasant and real. Coral reefs have multiple colors and look real as well. Graphics wise this game looks great and pulls off its theme of an underwater survival quite well.