I think it's the nostalgia in his books. He has a certain way of looking at subjects that makes them feel familiar and that we like. It's hard to say why we make that connection, but I think it's because he speaks in an honest voice. We know the good sides as well as the bad sides of characters, and we can relate to them in that way - they feel like complete people.
I think in general the characters are driven a lot by fear, more so than other emotions. But somehow that's the view he has on life - I think he looks at the world essentially as a place that is threatening, and where people get hurt: by each other, by diseases, by other threats. And his books are a lot about how people deal with these threats. I think, as Joe Hill said recently in an interview, it's a rehearsal: we know we can get diseases and will eventually die. Fiction gives us way of dealing with these facts, but in a playful, fun way.
I think he also has a way of planting things in his stories of something that's to come later on in the story. Sometimes he will specify shortly what is gonna happen and you will wonder how it's gonna play out. And at other times it's just a suggestion, where you're not sure what exactly's gonna happen, he's only hinting at it. This way he keeps you invested in the story, because you know at a certain point these things are gonna play a role, you just don't know exactly what effect they're gonna have.
And I think compared to other writers, he creates a richer, more real feeling world. You can relate directly to that world, because it feels like your own. So you get interesting characters and ideas in a world that you know, the only difference being that there are fantastic and supernatural things happening that you won't see happening in your own world. So, you get to experience a realistic feeling version of what could happen if something supernatural really would take place in our world and what the consequences of it could be.
All these things together make his work really readable.