For those of you who are not in the know, I'll take a little time to explain what game we're talking about here.
Anyone who's played the Civilization games knows that one of the eventual endgames is to construct a spacecraft and settle a planet orbiting Alpha Centauri. Sid Meier's Alpha Centauri is the story of what happens next. The settlers propelled away from an Earth that is steadily collapsing at the seams, dreaming of a more unified future for humanity. However, the settlers are divided into factions after an accident aboard the space craft, and each independently commandeers colony pods to settle scattered regions of the new world, Planet, on their own.
The game should not be thought of as 'Civilization: In Spaaaaaaaaaace', however. There are many, many more layers of depth. Rather than unlocking new units as technology progresses, you unlock new body types, weapon types, and special abilities and any unit can be equipped with any combination of these, leading to the player driven creation of extremely specialized units. There are no mountain tiles, rather the terrain has a hight map, so that a colony can be built on any tile, including ocean tiles with the proper technology, and the size of mountains and hills effects weather patterns. Elevation also effects the amount of solar energy you can harvest.
The most interesting new dynamic is, of course, your interaction with the native wildlife.
Another distinction from Civilization is that there is a storyline about the transcendence of humanity to be followed from Landfall, to the eventual endgame, should you choose to look into it. A lot of Nietzschean philosophy thrown about. There are also distinct landmarks scattered across the randomly generated planet, and finding them is useful for various reasons. The Alien Crossfire expansion expands on everything I've mentioned.
Also, the flavor text and voice overs for all of the technologies is extremely good and terribly interesting.