I fricking love Alpha Centauri, and I love Civ4, and I have spent (wasted?
) immense amounts of time on both of them.
When it was released, SMAC was undoubtedly the best of the civ-type games. Civ3 was undoubtedly a massive step backwards.
But Civ4 really is good. It's really as if they took a really good look at what works and what doesn't in these sort of games and really went to town on nailing them. Civ3 was oversimplified almost to the point of having One True Path, yet riddled with fiddly little things, so getting better was very much a matter of being more finicky with tedious micromanagement chores. Civ4 is pretty much as complex as SMAC/X (if not more so), it's just done in a more streamlined manner. Like SMAC, you can get better by delving deeper into its complexities.
That said, SMAC updated to take advantage of more recent developments would be mind-blowingly phenomenal. The setting, the factions, the ideologies, the cool tech, the fricking
nerve stapling, the terraforming, Chairman Yang, etc etc, it's all way cooler than "a history of the world".
AI: Easily the biggest thing Civ4 has over SMAC is AI.
Let's face it, SMAC's AI is lousy bordering on atrocious - terraforming AI in particular (so frustrating, since it would be so easy to improve!). On the other hand, the AI in Civ4 BtS is absolutely phenomenal (though even the regular Civ4 AI was leagues ahead of SMAC). It develops well, it techs well, it expands well, it trades well, it fights REALLY well, and it's capable of being an amazingly ruthless opportunistic bastard. It's something else.
Maintenance: For me, Civ4's biggest innovation was managing to stop REXing in its tracks. More territory is still better, but the crippling maintenance costs early on mean that it's not just a race to see who can make the most settlers in the shortest time. As the game goes on, more land is better, but it's perfectly valid to stay small early on, and the colonisation process goes on for much longer, which I reckon is much more fun. It also stops the whole thing of having a massive grid of indistinguishable cities one square from each other, which I don't reckon is much fun.
And unit maintenance via civ-wide gold rather than production in a particular city is a much better way of doing things, and saves a lot of pointless micromanagement.
Air power in SMAC is too dominant, and I think the Civ4 air-war element is done much better.
Unit Workshop vs fixed units with promos: I don't think one is better than the other, but I think both work better within their respective games. Fully-customised ancient armies just don't really make sense as much as they do for more futuristic armies (when everything was really dudes on horses vs dudes with spears vs dudes with bows etc). Fixed unit types denies you the strategy of making units custom-made for your campaign, but on the other hand there's a lot of strategy in figuring the best way to work with the constraints of what you've got (especially now that units in civ4 have more specialised bonuses and penalties). And the promos are a cool way of really customising units for specific tasks (often to a greater extent than you can with the unit workshop) and also giving individual units a bit of personality. And Unique Units are really cool.
I suspect there's maybe some amalgam of the two approaches that would give the best of both worlds without being too overly micromanagement-y, but until someone figures it out, they're both pretty good
Difficulty levels: I find this complaint very odd. I've beaten Deity on Civ4, but it's very very hard. There are people who find it pretty easy, though. If there was another level above Deity that was even harder, I'm really not sure why exactly that would make the game worse. On the other hand, Transcend is really pretty easy to beat regularly, and I know a lot of people would add self-imposed rules just to make it more of a challenge. I'd like to know that however good I got, there would always be another level to challenge me, without having to resort to OCCs or whatever. And sometimes, it's just fun (and instructive!) to just see how long you can hold out against truly impossible odds.
Civics: I have to give this one to Civ4 as well, though I
really loved the massive penalties that some SMAC civics would give. But the Civ4 civics mostly have a bunch more character to differentiate them and really allow for massively different tactics depending on what civics you've got running, much more than SMAC. If they had larger associated penalties (rather than just maintenance and the occasional small penalty), they'd be even better.
Resources: Bonus resources are really cool, and really add a level of depth to the game. It really helps differentiate different bits of land, too (this was a bit of a complaint of mine with SMAC - there were the occasional resource bonuses, but even prior to the advanced terraforming options, there wasn't as much differentiation of city sites).