This is the Civ game we've all been waiting for. All the best aspects of the previous Civ's and Alpha Centuari are here.
On the surface, the game starts just like the others. Your drop your first city and immediatly begin to explore and clamor to expand to the resources and land around you. The game progresses like you would expect it until you build your first worker. This is where the game takes it biggest turn from the Civs of the past. With workers, you now have a vast array of choices. There are a wide variety of things you can build. These choices only expand as you research techs that open up more options to your workers. I found the biggest problem for me was knowing what to build to best help my city. You can no longer just build a mine on grassland and irrigate/mine plains. The formulas of fthe past will not work in Civ IV. The problem is that some of the early buildings and improvements seem like a waste of an improvement. Fortunately, later techs and civics will increase the value of these improvements, but without that prior knowledge, its very difficult to know what to build. One key feature is that you no longer need roads on every tile in your kingdom. Some resources require roads to connect them to cities, but most things simply require them to be inside your national borders for you to gain the benefit of the tile. Having nearly completed a full game now, I can see the later benefits of the improvements and I have more confidence in what I build, but I definitely think it really takes playing through a whole game before you really get a grasp of what improvements to build.
Another aspect of improvements is that you see some specialized squares like you saw in Alpha Centauri in that one square will end up producing as much as six or more food. If you have a special food (corn, wheat, pigs, cows, etc) on grassland and farm it, you could end up with a high food producing square. You then need to think about balancing that with a high production square which you then have to balance with a high money square. In Civ III, you did not have this concern as every square produced at least some of the other. In Civ IV, you really have to plan out a city and make sure they are producing all three things in relative balance. This is another aspect of making it difficult to know what to improve and how to improve it.
This leads to a word on automating your workers. There are three ways to automate workers: Full, Nearby City, and Trade. Full automation lets the worker do whatever it pleases wherever it pleases. Nearby City focuses the worker's effort on the city it is near until it is completely upgraded, then it moves to the next city. Trade automation has the worker focus on building roads to connect your cities and building improvements to take advantage of the special and strategic resources within your borders. Trade workers will even go idle when there is nothing to do, but will spring into action when you build a new city or discover a new resource. Of all the types, the Trade automation is the best in my opinion. A couple of these guys will help keep roads built between cities and immediately take advantage of new resources. Otherwise, It's best to learn what tiles can be improved in what way, and then choose how you improve your land yourself.
Otherwise, the game is very straight forward. Combat is simple since units only have a strength score. Upgrading units is easy, jus refer to the poster with the upgrade chart and pick your upgrade poison. Civics are easy since you gain access to them gradually and have plenty of time to access how they affect your nation. Diplomacy is easy, research is easy, happiness is easy, the new food/health system is easy. The good thing is that they all add up to a very diverse way of playing the game.
One of the big questions and concerns is the religion thing. Trust me, it does not push one religion over another, there are no advantages to having one religion verses another. In fact, later in the game, one strategy and benefit is to have as many religions as possible. It is simply another layer of depth to the game play that adds variety and more wonders to build.
Let me sum up by pointing out the different aspects of the game and where they were first introduced in my opinion.
Cities and expanding: Civ I
Worker improvements: lots of new improvements specific to Civ IV
Unit upgrades: Civ IV
Combat experience: Civ II
Collateral damage in combat: Alpha Centauri
Diplomacy: Civ I
Civics: Alpha Centauri
Culture: Civ III
Religion: Civ IV
Tile specialties: Alpha Centauri
Railroad movement: Civ IV (finally its realistic only 10 tiles/turn)
Selective technology tree: Alpha Centauri
Demographic information: Civ I (was gone in Civ III)
Top 5 cities: Civ I (was gone in Civ III)
Wonder list: Civ I (was gone in Civ III)
Wonder Movies: Civ II
Great Leaders: Civ III
The point is, I really feel like this game has finally taken the best aspects of all the Civs and Alpha Centauri and has combined then into one GREAT game. The additions of some fresh content and strategic depth is great, but I would have been happy with just the compilation of past features.
Regardless, This game is great. Once you play through a game and get a feel for how develop your land tiles, this game is nothing but smooth. There is no longer one way to win or one nation that is the best. I get a real sense of balance and fairness to the whole game. Get it play it and have tons of fun.