I love subtle details you can perceive if you’re paying attention, but aren’t exactly buried too deep. You feel this sense of satisfaction because you noticed, but it’s as subtle as what the author did, not a huge aha moment.

Examples: in Handmaid’s Tale you can deduce the protagonist’s name. It’s never stated but it’s clearly there if you pay attention.

Also: in Remains of the Day how you notice things that even the protagonist is oblivious to - though I’m not sure if it counts when it’s the whole point of the story.