I’ve played quite a few games, from Mechwarrior to Tetris, and very few have posessed me to come back after beating it and play through again.
IW2 has IMMENSE amounts of single player replayability. I’d forgotten how much fun pirating was until I got back in the tug and started laying down the law.
Physics is another aspect I love about the game… While we shouldn’t be able to hear anything outside the ship, the rest is actually fairly accurate. Such as acceleration, deceleration, inertia and whatnot.
Lastly, I think the sheer vastness of the game is quite appealing, there’s nothing quite like looking at the distance to target reading and noticing that you have 2 billion kilometers of space between you and where you want to be. I’ve tried getting some high speed with thrust override and that was fun too (moving at 40 km/s is very satisfying when you know you ain’t using LDS).