One of my dislikes is the term sci-fi. It was coined with negative connotations and still has them, as far as I'm concerned. But that's just a pet peeve (along with the entire gaming industry's mis-use of the term 'fog of war').
I like my SF games to have a fair chunk of science in them. Traveller's my favourite, for all the reasons Shadowdancer mentioned, though I never liked the Imperium that much or the game's militaristic bias. (My solution was to flesh out the classes in Supplement 4: Citizens of the Imperium, so that they were as viable as the expanded core classes.) I like starships but I'm not a great fan of starship combat. I like sentient aliens but the more alien the better. Vargr, Aslan and Zhodani never really grabbed me, despite the rationale behind them. Firefly succeeded in bringing back memories of many a classic Traveller game.
Despite all the things I've just proclaimed, I like Chaosium's Ringworld game. That's largely because I'm a fan of the Niven books. The game system itself could have done with more development. I'd still play a Traveller or Ringworld game now, if I could find anyone to run one.
And in spite of my disdain of science fantasy, I was hooked by Skyrealms of Jorune. The game system was a joke but the setting was as evocative as it was eldritch. I liked it because it was like nothing else.
Hussar's point about SF games not keeping up with real world tech is spot on. Traveller's computer rules looked shaky from the moment they appeared in print. I remember listening to a talk by Bob Shaw at an SF convention many years ago, in which he addressed this issue by way of recalling an old episode of Flash Gordon. Flash's rocket ship is going to crash and the crew are bailing out. Someone tells Flash to get a ray gun. Flash picks up a bulbous, silvery ray gun. Fair enough: who knows what handheld ray guns will look like? Then, instead of a parachute, Flash is told to grab an anti-gravity belt, so he puts on a wide, silver belt. Fair enough again: who knows what such things will look like? Then someone says, "Don't forget the radio!" Flash equips a backpack-sized silver box with dials and a circular antenna - because, in the nineteen thirties, everyone
knew what a radio looked like!
Another difficulty with SF games is that of inconsistent tech. The writers of Star Trek in particular struggled with this for years, as they became victim's of the show's success. David Trimble discussed it in his The Making of Star Trek (great book - don't know if it's still in print). One week the crew of the Enterprise solve a problem by (a) using tech x or (b) discovering tech y. But a few weeks later, the cast suffer from mass amnesia when these miraculous technologies are mysteriously unavailable, despite presenting ideal solutions to the latest challenge to face the crew. The fact is, the writer of the later episode is unaware of what has happened in the earlier one. In more recent Star Trek spin-offs, the writers became more sophisticated, in that they would often try to provide the audience with justification for the unavailability of last week's solution. But the contrivance is more pronounced and the exposition is just a bit more painful for anyone who's followed the series closely.
And this is the problem I think referees of SF games can find themselves facing very quickly. After all, the introduction of exciting new tech is a compelling adventure hook. The trouble is, once you've let the all-singing, all-dancing nanobots out of the bottle, it's very difficult to get them back in without destroying your players' suspension of disbelief. But introduce miraculous tech into your game and it risks spiralling out of control.
If I were to run another Traveller game today, it would have, among other things, the following characteristics:
Very few sentient races. If humaniti were at TL 10, I'd have the other races at TL 1 and TL 20.
Any advanced alien tech would be extremely dangerous and utterly beyond the comprehension of humaniti. PCs would find it difficult to retain possession of such tech.
New computer rules.
I confess to having missed Transhuman Space. It sounds like a game I could get along with.