Magic items.
A key feature for me in RPGs is the possibility of acquiring "magic items", by which I mean items that grant special powers...making my character more powerful in some way...and cannot be acquired in any deterministic way e.g., purchased from a store, granted as part of regular progression, etc.)
In a science fiction game these sorts of things can be (and often are) not "magical" but created using forgotten technology from an extinct race. Which (Arthur C. Clarke reference) is really just magic. But as soon as it becomes "just save up enough credits and you can buy one" then, pardon the play on words, the magic is gone.
So I would have trouble enjoying a strictly "science fiction" game that didn't somehow incorporate some analogue to magic items.