Evolution of Gameplay. It's not the most important thing, but it is one of those things that separates good games from great games.
Alpha Centauri (AC) is a prime example of this.
In AC, the gameplay changes. Technology doesn't merely give you "bigger numbers" (Factory 2, which is better than Factory 1, etc). Certain technologies fundamentally change how you play the game.
Take the basic economic model of land improvement, for example. In the early part of the game, the ability for Terraformers to plant forests is worthless. Forests may fight Xenofungus pretty well, but they're utterly useless for providing your cities with vital resources. You're better off laying down farms, mines, and solar collectors.
Well, until you start hitting the, for want of a better word, forestry techs. These provide buildings for your cities that allow them to get good resources from forests. Now, it's never quite as good as it would be if you properly manicured the land with boreholes and so forth. But it provides a good balance to all of your cities, and forests tend to grow on their own.
Later on, you get the ability to build satelites that beam resources to all of your cities. This fundamentally changes how you approach economics in the game. Rather than improving land, it's more efficient for you to build large numbers of satelites of each type, and let the resources fall from the sky. The downside: you have to dedicate a lot of production to it over a long period of time (much like building Econ starbases for their production bonuses in GC2).
The basic economic model, the foundation on which the game is built, radically changes over the course of the game.
Something similar happens with units too. In the early parts of the game, you have ground units. You can make units that move 1 movement per turn, or 2. This is an important choice, since units with 2 movement can retreat from a battle if they're losing (and they have a movement point remaining). However, it is very costly to build a unit that can move fast, attack hard, and have 2 movement, so there's still a reason to have 1 movement units around.
Then, things change; the status quo is overturned. Resting at the apex of the triad of victory is Doctrine: Air Power. Once you get that tech, suddenly you have flying units. They bypass terrain. They can travel long distances. But they can only attack once. Air-to-air fighters must return to a city on the turn they launched, while bomber-type air units (can't attack other air) attack once then have to return to a city on their next turn.
This fundamentally changes the flow of battle. Approaching a city with your ground units is going to be much more difficult if your opponent can take them out (or just wound them) with air units. And if one of their cities is close enough, he can strike at one of yours directly with his bombers. There are counters to these, but they can be somewhat expensive, and all of them require Doctrine: Air Power. The only saving grace is that air units cannot take cities; you still need ground units for that.
Later on, you learn a tech that provides you with Copter air units. Copters are unique air units in that they can attack multiple times per turn (one per movement remaining). They also do not need to return to a base at the end of the turn. But if they don't, then they take one third their maximum Hp in damage. Copters are devastating when used properly, as they can do more damage than an equal number of bombers. Suicide copters can be very dangerous.
Beyond that, you can get drop pods for your ground units. This allows the to, if they are in a base, suddenly jump to a location fairly far away. With later technologies, you can jump them to any square on the entire planet. Cloaked ground units can do this too, and nobody will know about it until you roll your GravTank up to a city that thought it was safe and say "Hi."
And after all that, you get the ultimate unit movement type (well, almost): the GravShip. It's an air unit that does not need to go back to base at all, and it can take cities. It also has the largest movement in the game. It doesn't have the ability of the Copter to attack multiple times per turn, but you can bring a few Copters along to soften up the defenses.
More 4X games need to do this kind of Evolution in Gameplay in their games.
