People ask this and I often give a bad answer, so here's my best attempt at one!
There's loads of things but right now two things come to mind. The first is that I want to see what people are like when it's actually on. Not just a bad week, an actual wall... The road home stopped working, the air budget, the ship that came back having lived more time than it should have. You don't find out who anyone is at dinner. You find out when the margin's gone and they still have to decide. It's easy to be good and be the person you believe you are when life is simple. When the world around you is collapsing and there are serious stakes and consequences, can you live up to the person you believe you are?
Science fiction lets you build that wall as high as you like and keep it honest, because the physics says what's possible and the physics doesn't care about what anybody thought they were.
The second is the rules. SF is the only genre where the rules of a place get to be part of the story. A crime novel takes the law as given... A literary novel takes the family as they are. Here you can build the institution, the economy, the clause somebody wrote in a hurry twenty years ago, and then put a person in front of it on a clock. Kim Stanley Robinson writes novels where the big scene is a meeting and I find that more tense than a gunfight. Le Guin wrote about what a society owes a person and what the person owes back, and never once let either side off.
What I don't want: chosen ones, and an enemy who's evil so nobody has to think. I'd rather have a warship captain who's right, mostly but being right will still cost him.
So that's mine. Why this genre and not another, and which book/series/movie made you fall in love with the genre?