The sound, music and ambience. You will get pulled into it, and for at least a week, it will consume your life. And after that week, you'll find out there are multiple ways to play it, and you'll never want to stop until you do it all. Guaranteed.
Oh and there's sex, strippers, beautiful bald women, aliens, guns, biotech, excellent dialogue, multiple planets to explore, traitors, your own spacecraft, and one epic story. You need this game if you're a action/RPG fan.