Thinking about science fiction settings in rpgs (and in film and television which I think is the biggest influence on rpg sci-fi settings) I think that two important factors are scale and frame. Scale is the size of the setting, not necessarily in absolute terms (though maybe), in narrative terms. Frame is a descriptor or genre of the typical types of stories the setting supports. The two factors are not independent or exclusive.
- Crime/Hard-boiled Mystery (Outland) - Hard people doing hard things...in space
- Exploration, Pulp (John Carter) - A stranger meets a strange land or lands
- Exploration, Mystery/Horror (Alien) - we've found something anomalous and now it might kill us.
- Exploration, Realistic - (can't think of film here) - Alien planets are mostly inhospitable, talking to other species is hard!
- Exotic Ports of Call (Star Trek) - every week another world, another adventure
- Outpost (Babylon 5) - Everybody comes to Rick's
- Pioneers (Earth 2) - A little bit of exploration, but mostly we're putting down roots
- World-trotting (Star Wars) - Constant motion; as many exotic backdrops as possible
- Galaxy Wrecking (Guardians of the Galaxy) - the universe is vast and wild
- Ship/Station
- Planet/Megastructure
- Orbital System (this could be either a group of moons or artificial satellites)
- Solar System
- Near/Few Star Systems
- Several Star Systems
- Many Star Systems/Galaxy
- Galaxies+