Of course you need good characters and a good plot, but there is another element of fiction that writers often overlook. And this is one that readers often appreciate and remember.
I think there are two routes for the writer. They can provide ideas to think about and/or they can teach their readers about a subject. In the first instance, this can include raising questions that provoke the reader to think about a topic. In the second, the author can explain an esoteric subject. An example of the first instance might have a character facing death. Fine, that is more than enough to hold your reader’s attention, but if you want to really intrigue them and make them think, speculate about the subject of death. What does the character believe happens after death? What other ways of viewing the afterlife exist? What does it mean to not believe in an afterlife at all?
Or on a less morbid subject, perhaps your character stops one night and stares at the stars. For no reason other than he looked up that moment, he finds himself thinking about the infinity of space and man’s insignificance in the universe. This sort of speculation gives the story more depth and can capture a reader in a deeper, more lasting way than your story itself might.
The other route of involving the reader more deeply and memorably is to teach them something. The subject has to be relevant to the story, of course, and it has to provide new facts for the reader, i.e., something interesting that they likely did not already know. So a character visiting an art museum might come upon a famous painting by Goya and think back to his college art history class. The character can think about what he learned in school about Goya’s relationships with the women in his life. Sharing details with the reader about the artist, how his love life influenced the painting, would add to the reader’s knowledge and positive experience with your book.
So think about exploring wider ideas or teaching your readers something.