And for me, sci-fi is all about a warning or a thought experiment into possible outcomes - "if you do this, then that might happen". And "this" could vary through scientific breakthroughs, encounters with alien life, or just different ways of running a society. The important part is how society forms itself around these scenarios.
Of course the running-down-corridors-shooting-things kind of sci-fi involves these events as they first take place. But the more sophisticated sci-fi comes from how society stabilises with these things in place, and this all comes from the setting which is there but is so omnipresent to the narrative as to be almost invisible (even if it occasionally needs describing so we know what's happening). The burbclaves in Snow Crash, replicators in Star Trek, phone as prosthesis in Halting State, enforced pacificism (or else!) in Day the Earth Stood Still, long-term alien settlement on Earth in Alien Nation and District 9, and so on. So Aliens makes it as sci-fi on the grounds that the Marines aren't there to save the civilians or kill the aliens, they're there to capture and return them for experimentation and use as living weapons.