1) Three little leaflets ("books") of around 50 pages each forming a complete RPG system.
2) Character generation is a game by itself.
3) Easy to learn, easy to use, easily extendable ship design system.
4) Quick and dirty, "brutally" simple, "brutally" elegant and "brutally" efficient rules for everything from chargen to combat to wounding. By "brutally" I mean a system that does not try to be sophisticated, it just achieves its effect without fussing around. And, for the most part, it works.
5) A pure skill-based system in the age of class-based, level-based systems.
6) A generic system, yet with a few distinguishing features (such as the speed of travel/communications) to help you focus your universe-building efforts.
7) Meshes well with many sci-fi classics. How many alterations will you need to do to CT, for example, to play in the Aliens universe? Somewhat better computers, "androids" that will probably be treated as "humans" rather than LBB8 robots, VERY limited FTL communications (if any) and the need for low passage on long trips. JTAS even has the stats for the xenomorphs!