A good science fiction adventure
Well, it depends on the subgenre of science fiction you're talking about. Cyberpunk should have intrigue, noirish urban atmosphere, and interesting technology, whether in cyberspace or as potential gear/enhancements. Lethal combat, potential backstabbling, and a dystopian worldview are all par for the course here. Postapocalyptic settings share some of these characteristics, but with weirder mutants ("Shadowrun" blends some fantasy into all this).
Space opera should share some of the sense of wonder familiar to fans of high fantasy. Heroic missions, spectacular space battles, varied planets and aliens to encounter, and a good pace/party dynamic are key here. Swashbuckling personal combat, low Player Character mortality rates, and a somewhat lighter, more romantic tone are common in D20 settings such as Star Wars, Farscape, Stargate, and Prime Directive/Star Fleet Battles/Star Trek. Each of these had some useful support.
"Hard" speculative fiction (such as "Traveller" or "Firefly/Serenity") should involve exploration, resource management, and careful maneuvering for long-term gain. The tone of the game can range from grim and gritty (like cyberpunk or the Old West) to antiseptic (but no less dangerous; see "2001: A Space Odyssey") to somewhere in between ("Blue Planet"). A smaller scale -- such as one or two planets in the not-to-distant future -- is also more likely than a galaxy-spanning epic.
An individual scenario should challenge each Player Character in different ways, whether it's the cyberpunk fixer trying to get some information out of a reluctant contact or a hotshot fighter pilot trying not to get shot down over some uncharted planet. The best adventures reward the entire team for its efforts. Exploring ancient ruins (easily ported from D&D), conducting a diplomatic first-contact mission (see "Trek"), or winning an interstellar war can all take multiple sessions. The old "Star Frontiers" game and D20 "Babylon 5" 2nd Ed. are excellent examples.
I myself like a mix of planetside "wilderness" adventures, urban (or starship-based or space station-based) intrigue, and classic "dungeon" crawls through enemy ships or bases. Bizarre mutations, mindbending substances and psionicists, and a wide range of gadgets and hostile aliens should allow for endless possibilities...