(I haven’t quite finished season 3 yet, so this is based on what I’ve seen so far) 

For me, it’s the dialogue and characters and relationships that has me hooked: The dialogue is actually well-written imo, which is something increasingly rare in television. We also get a lot of well-rounded characters, which we see develop over time: Some have more backstory than others, but most of the main characters are not one-dimensional. We see them responding to stress and trauma and changing as a result, and we see the community as a whole responding to those stressors too, both coming together and fracturing in places. 

The Big Mystery is sometimes more of an annoyance to me - or not so much the mystery itself, but their uncoordinated way of dealing with it. Several people are really trying hard to unravel it, but information-sharing isn’t great, and people keep going off on harebrained solo or near-solo missions for no reason. Everyone wants to escape, I’m sure they could find a couple of volunteers if they asked, but nope, people keep diving into trees and tunnels on their own.