The flexibility. Hands down. In other games (like RFTSII or STUN) you can play around with the percentages somewhat but you are pretty much stuck with the stock ships and races. With SEIV, the sky's the limit. New shipsets, new races (with culture, speech, research, ad inf.), new graphics and sounds (I
immediately replaced those silly warp points with B5 jump gates
)... and that's just playing with the base files. Get into true mods and you've got a game with darn near infinite replay value.
And the way everybody sings the praises of PBW, I may just have to get off my duff and get started over there pretty soon...