The setting, the atmosphere, the worldbuilding, the ambience, I personally enjoyed all of it. The game does a great job of constantly making you feel unsafe throughout the tunnels and on the surface even when with a group of allies, and only let you breathe easy in few situations.
The minor characters, while at first it felt that they would be throwaways, they each have a well established personality, even some unnamed enemies you can listen to occasionally.
I did get frustrated at certain points in the game where it felt that I was forced to stealth and sometimes the stealth felt inconsistent (Frontlines).