Also see GURPS Adaptations
Is the source distinctive (Babylon 5), generic SF (Star Trek, Poul Anderson’s High Crusade), ‘historical-era plus’ (Conan, Harry Turtledove’s Colonisation series) or ‘present-day plus’ (28 days later, Dark Angel)?Edit
For a distinctive source, the GM needs to inject a lot of detail in their narration which captures the feel of the source. Any Babylon 5 adventure must mention the cramped spaces and the concentrated presence of lots of different alien species. Generic SF is easier as anything goes – just be careful to keep it consistent. Historical-plus (steampunk, pulp fiction (the genre, not the Tarantino film!), alternate-history) allows you to use the plentiful references in history books, and ‘present-plus’ is easiest because many of the resources are all around you.
You may query my choices of examples: I reckon Babylon 5 is distinctive in that almost all of the action takes place in one small location – a space station. Star Trek is generic because although there are things players will expect to see – Klingons and the Prime Directive – they are just window-dressing, and the players will be probably be running around the galaxy having very similar adventures to those in a High Crusade campaign (where the PCs’ primitivism is just another ‘special effect’).
How much scope is there for adventures?Edit
Some sources are constrained. Base a game on Primeval or Tremors, and the scenarios will be ‘monster appears, players hunt it down and deal with it’. Both would be fun for one-off adventures, but with less appeal as a campaign theme. Most SF or fantasy worlds are much more open to expansion in any direction the GM and players want.