You have told the whole true, guys...
1 social engineering is a really perfect idea - we can construct our state anyway we want during the game. in civ there is only one factor [gobernment], in SMAC there are four factors, and because that the game is more complicated more playable and it cannot make you bored even after days of playing, because every game you can play in a random way...
2. great story [one of the best sci-fi stories I've heard, Star Wars, Star Trek etc. suxx in comparison to SMAC], great climate, great quotes [both said by game authors and philosophers]
3. one of the best AI in the game. I won't forger when I played Greens, and Lal was on the other island. i had much more ships than he so he raised terrain to reach to me and crushed me out... the AI is not only "just very good", but also more realistic than in civ
4. very well-thinked factions - every is different from the other one, playing Gaians is a completely other ecperience than playing eg. Hive...
5. PSI forces - second type of attack gives you more tacktics, more strategies and greatly improves the climate. i love the tick of mindworms spoiling the brains of enemies...
6. new strategy possibilities - fungus where the units can hide, formers which can raise terrain and let you go to enemy territory without ships, melting polar areas and flooding enemy bases and many many more...
6. great terraforming, much more complicated than in civ and far more interesting, giving you much more choices and possibilities
7. nice intro and wonders films...
ok , that's enough...
IMHO SMAC is the best game ever made, the game just rocks, I will play it forever, I'm sure of it...