I think one of the things I find most attractive in any novel is the feeling that it is telling me secrets about the world I live in. You see this simply and pretty clearly in a lot of middle reader fiction: Hogwarts could be in real England, Camp Half-Blood could be in real Long Island, the Mysterious Benedict Society could be solving real logic puzzles (ha!), Wildwood could be set in real Portland, etc.
It's also, for me, the lure of historical fiction. Thomas Pynchon's Mason & Dixon, for instance, or most of Umberto Eco's novels, or Michael Chabon's books, tell stories that might have happened in the real world, and because they might have happened, they act like a sort of mirror held up to the real world, to help me interpret what really did happen. I'm told a fake secret to get to a real insight.
It's also the attraction of the best speculative fiction. J.R.R. Tolkien (than whom none greater), for instance, consciously wrote a sort of invented mythology for England, making The Lord of the Rings a mirror held up to the history, culture, mind and language of the Anglosphere.
What's the secret world in your novel? What insights are you sharing with your audience? What mirror are you holding up?