For me, it is all of the little details that make the game fantastically immersive and fun.
The coms from the ship captains when they prepare for jump and when a ship is built are excellent. The pirates comments are fun and they make you want to go after them.
The graphics all have little tweaks that make them more interesting. The waves of missiles heading from a battle group to a target, the engine trails, how the bombers and fighters attack then veer off to line up another attack run. These are all details that I have discovered and that draw me in like I'm reading a good book. I keep zooming in and out of battles so that I can discover more.
IMHO, the more of these little details you can put into the game, the more immerzive and interesting it will be. Don't misunderstand, I am already more than glad that I spent my money well with this game (hey, with GalCiv I and II, how could I have gone wrong) but more little details, auditory, visual, and otherwise couldn't hurt!