One of the big differences between Fantasy and Science Fiction RPing for me is that SF settings tend to be extrapolations from our world. It’s most-commonly implicit that this is the future, it’s derived from where we currently are.
Yes, I recognize that I’m setting aside Star Wars, which says “A long time ago, in a galaxy that’s nowhere nearby” - - I consider to be its own genre, or at least just thinly veiled fantasy, because: SPACE WIZARDS. To some degree, Star Wars is basically its own flavor – it’s what happens when a 1970s conception of worn down, lived-in technology becomes inherent in the aesthetic, even as the future has surpassed it in many ways.
Personally, when I think “SF RPG,” my mind goes to Traveller first, and nearly immediately I feel limited. Not only am I looking at a (largely) human-based, a laws-driven society, where assault is a crime, and murder even more so... and I feel like I want to go back to bashing in kobolds’ heads or dropping a fireball on the Evil King’s horde of brigands. IT’S SO SIMPLE, rarely repercussions from the local law enforcement, telling me, “Hey, we can see why you hit him, but that’s still Assault. Your turn in court will be on Monday. Enjoy the weekend in hoosegow.” Sure, GMs run looser law enforcement in Traveller, or even tighter/more restrictive faux-medieval guards in fantasy, but it’s just how my head works.
MORE THAN THIS, technology feels like it has to have logical consistency. When someone who understands an engineering discipline better than I do (which is basically everyone who’s taken an Engineering course) takes tech in the world and wants to know how it works, so they can better exploit it. This RARELY happens with spells. So part of my aversion to SF roleplaying is not having the engineers running circles around me in abusing equipment or networks, and circumventing the obstacles I’ve placed before them.