Although the game has always been addictive, and popular, it suffers from some substantial defects.
It's a lousy wargame
Civilization is more than just a wargame, but warfare is a significant part of the game.
The Civilization model of combat is that each military unit is one person, and all combat consists of a one-to-one fight between one of your units and one of your enemy's. Most often, combat is to the death and only one unit survives; cavalry units may sometimes retire hurt.
This is totally unrealistic and has unfortunate practical consequences, as I'll explain.
The wars in this game are supposed to represent combat between nations. As such, each military unit should represent thousands of people (at least). Such units very rarely get wiped out in combat. They take casualties, they retreat if defeated, and in due course they get reinforced with new recruits.
Furthermore, if a bunch of my units attacks a bunch of enemy units, this is one battle and should be resolved as such. In reality they don't line up and take turns to fight each other one-to-one.
Civilization's absurd model of combat is also very tedious for players. Because units keep getting wiped out, the player needs to be constantly creating new units and manually moving them one by one up to the front line. Because combat is one-to-one, players often have to fight a long series of battles for a single position in a single turn, which should be dealt with more quickly as a single battle.
If units were correctly seen as consisting of thousands of soldiers, in time of war most of the new recruits generated by your nation could be sent automatically to top up existing units, with no manual intervention from you at all. New units would be created only when all existing units were at full strength. This would give players much less tedious work to do.
It takes too long to play
A full game consists of hundreds of turns, in each of which players have to contend with a lot of micromanagement decisions, most of which are rather tedious and repetitive. And you have to slog your way through all this in order to get any result from the game. I'd much prefer a shorter game that emphasized the important strategic decisions and cut out the tedium.
A game should be a brief, pleasant diversion from real life. If a game starts to take over your whole life, something is wrong. But a game like Civilization seems specifically designed to take over your whole life for long periods of time. Game designers actually seem proud of this effect: they're delighted that they've managed to turn large numbers of people into useless zombies.
Corruption and waste
The game simulates corruption and waste by making cities less productive the further they are from the capital and the more cities you have in total. When you have a large empire, its remote cities are in effect useless parasites that have to be supported by the core cities.
This may be somewhat realistic for ancient civilizations, but the game makes no allowance for improvements in communication and transport, so I think the effects are overdone in medieval and modern times.
Certainly it's very dispiriting for players that, the more successful they are in expanding their territories, the more dead weight they have to carry. Remote cities require care and attention, this takes up the player's time, and it feels so unrewarding. It's just not fun.
Some are more equal than others
Because of the randomly-generated map, some starting positions are significantly better than others, which means that the game is unfair (in any particular game, the various players don't have equal chances), and it also means that you can't compare your achievements in one game with your achievements in another.
However, the random map makes the game more interesting and fun, so I don't know what can be done about this.
Specific rules changes
Here are some specific things in Civilization III that I think are wrong and should be changed.
- Each military unit should be treated as a division consisting of many people, which takes casualties in battle but rarely if ever gets completely wiped out. If defeated, it should retreat. Reinforcements should go automatically to replace casualties in existing units; new units should be created only when all existing units are fully topped up.
- Units should be upgraded automatically when new technology becomes available. Maybe this should cost something, but it should be done. It's ridiculous to have spearmen still running around in modern times.
- Units should contain a mix of weapons, including artillery. You can have separate cavalry or armoured divisions if you like, but no separate artillery divisions.
- If military units enter a foreign country without specific permission, it should amount to a declaration of war. I think this should apply to settlers too: in real life we don't see large organized groups of foreigners wandering around our countries looking for places to build their own cities.
- Ancient units should consist of smaller numbers of people and should be capable of living off any patch of land that produces food. Unit size should increase with technology (helping to keep down the unit count in modern times). By the time technology yields firearms, units should need to trace a supply line by road or river to a friendly city. If no supply line, they disappear (they surrender or disintegrate).
- Building roads and railways should cost money as in real life (in real life they also cost money to maintain). Players should be discouraged from building roads on every available square: this makes the map look ugly and unrealistic. It also seems unrealistic that eventually almost every square is either mined or irrigated. Perhaps mines and irrigation should have a maintenance cost, or should be useful only where a specific resource exists.
- War weariness should be related to the number of casualties suffered abroad, not caused by the mere presence of military units abroad.
- The effects of corruption and waste should be somewhat reduced, and related to travel time rather than to distance. Perhaps hurrying production with cash should be cheaper. But get rid of the silly system of disbanding military units to hurry production.
- The use of civ-specific abilities and military units is absurd, and I always turn off that option. Indians used elephants for war because they had elephants and figured out how to use them in war. Anyone who had elephants could have done the same, as the Carthaginians famously did. It's quite a nice idea that different civs should have different characteristics, but this can be achieved with some extra rare resources (e.g. elephants), some extra dead-end technologies that most players won't bother with, and some extra Wonders. Your own civ's characteristics should be determined by its environment and by your decisions.
- The game's finishing date should be easily changed by the players. I don't want to go on until 2050; I find the modern period unappealing and would prefer to stop much earlier. This would also considerably shorten the game, which I'd consider an advantage.