1) Customization (shipsets, graphics and such).
2) Mod-ability (changing the rules).
3) Roleplaying (download the SW Empire shipset and tell other races, "We do not negotiate with Rebel scum").
4) The community.
All of this is without PBW. I'll go there when (if !) I ever get bored w/ #s 1-4.
(BTW, I'm back. I was off playing other games for 3 months, but they only made SEIV look better).