I have a few absolutely lovely superfans. I adore them because they remind me to keep writing even when I don't feel like it. My fanfic has about 30 readers now, which is a really nice feeling, and I appreciate everything they say and do for me.
One complaint that I get from some of those fans is about a lack of detail-in-the-large. As an SF reader, I'm accustomed to pulling out the surrounding enviornment from hints and filling in details by noticing little things. The famous "The door irised open" line from Heinlein sets a stage; as does my latest opening line, "If you can see the space elevator from here, my Queen, your eyes are better than mine." But I know what a "space elevator" is.
One of the core conceits of the story is that we have two human civilizations that have just met: one is purely slower-than-light; the other has achieved FTL transport. The STL people use technologies that are probable, including terraforming solettas and space elevators. Some of my readers have no idea what those are. They also don't know what a wormhole is, or gravitic sheer, or a railgun, or an electromagnetic pulse. I'm really comfortable with these concepts, even if some of them are pure fantasy. Since I'm releasing the story serially, I've had to go back and help explain in more detail many of these concepts, often in an "well, we discovered phlebotinum in 2500 PD," or "We don't have your phlebotinum, so we had to do..." sorts of ways.
But fanfic readers want even more. The whole "show, don't tell; keep it short, make it punchy" thing that our editors have been telling us for years is utterly ignored by fanfic writers, and I don't think that's entirely due to they're all being mostly younger, mostly beginner, mostly immature. Fanfic readers want to be spoon-fed every detail; they want every emotion spelled out, illustrated in excrutiating detail, and then verbally reiterated. They want distance from and insight into the POV characters, and they want the writer to go over that ground again and again until they have perfect clarity as to where the characters are going. What fanfic fans they want, more than anything else, is to show each other how to process the world: how to process love, fear, betrayal, friendship, life, and death. The whole point of fanfic is to get inside the mind of beloved characters and answer the question, "Why is he/she like that?"
Chuck Pahalniuk once wrote that writers should avoid the characters "knowing" anything: never write that a character "knows" how another feels, or "believes," or "feels." To a fanfic writer, that advice feels like an abdication of what fanfic is all about.