Really, pretty much everything you could ask for from a space simulation game (except for multiple endings). I originally bought the game almost a decade ago, when it first came out, and finding it again on this site, decided to buy it again (my original CDs long ago scratched beyond all repair).
Actually, what really drove me to purchase the game was the FreeSpace Open project (hard-light.net), which expands upon the original game to make it something, well, extraordinary. Perhaps my rating reflects the original game + FSO, but as FSO is freely available, it still seems appropriate.
I don't know what else to say about this game beyond, "great." I'm one for combat-flight-sims, I enjoyed my Rogue Squadron (though this game is infinitely less impossible, with difficulty levels!), but the depth into which the game draws you into its universe is incredible (or maybe I'm just trying to say it's engrossing)... I've lost many weekends to this game in the past, and look forward to losing many more!