intriguing characters and relationships and exploration of things important to these people. a lot of my favorite books are romance novels, even though I barely read anything in that genre but it's pure character and relationship and if done well it's such a joy to read. Wuthering heights and call me by your name come to mind. I want to hear the characters thoughts, see them interact with each other and know what they feel towards each other. It's also great to see how characters deal with the elements of the story on a meta level. The song of Achilles is an exploration of what it means to be a hero in the mythological sense, what kind of person you need to be and why a pacifist would go to war, knowing he would die young. That's the coolest shit.

Another important factor is I need some kind of tension or mystery in the plot. when I can't even imagine what the next scene might be or what I would want to read, I can still enjoy a book but I will never have motivation to continue it. just characters rambling dorian grey style gets a bit exhausting.