The almost infinite replay potential, it's quite staggering actually. I sure hope MM is financially sound, I'd like to be still playing this game, reading this forum, and learning new tricks when Space Empires X comes out!
My favorite aspect of SEIV is simultaneous play against other human players. I like games that use the hosting model, not turn based where you pass the turn on to the next player. So many potentially good games have been ruined by not using the hosting model for turns.
I have lots of computer games that I do not play much because they are against the computer, turn based, or real time.