Suppose I make a version of CiV where I decide that every AI player starts with 1,000,000 gold, buys the best military unit they can afford (with a mix of settlers to expand so they can buy more military units per turn). And is very warmonger oriented. This game would obviously be hard (The AI would have such an absurd unit advantage and would convert that into attacking you, but it wouldn't be fun. The way in which the AI breaks symmetry is just completely jarring and boring.
In CiV, we have a tactical combat system where the AI can't adequately handle combat. As a result it recieves huge economic bonuses, which already tends to force strategies into a very narrow window (the easiest way to beat the game is to exploit the AI's bad combat interface, trying to keep up economically is suicide). Several strategies emerged for keeping up economically, most notably ICS, and the game programmers responded by nerfing ICS to the extreme for game balance (fine it was unbalanced), but also with an AI programmed to ICS (look at the AI city spacing), and then instead of adapting the AI to the nerfs, they rely on a massive happiness bonus to ignore them.
In essence on high difficulty levels the AI is playing a game of ICS with other huge bonuses (free units including a settler, two workers, four warriors,...). That game got given added value by making its cities more easily defended (another patch change), and by the results of taking cities as puppets (happiness penalties) resulting in it being hard to conquer the AI in one go. Instead of the AI offering me a tactical challenge, I still destroy it tactically rather effortlessly, but the happiness penalty exists slowly to slow down my steamroll. If a player had the same happiness bonus as the AI it would without a doubt be correct to ICS. In essence, the game has one clearly optimal strategy which the AI is using and a system (happiness) is introduced in a way that doesn't affect the AI using that strategy but which closes it to the player. This violates every notion of the game being a symmetric strategy game.
keep in mind that this is all only the case at higher difficulty levels. on king the ai doesn't get those huge bonuses and doesn't ics. on settler the human can ics with his huge happiness bonuses as well. if you feel like the ai is cheating then you must not remember cIV very well, remember the deity bonuses in that game? 50% + of the time you were dead on deity at turn 1, you were just a walking corpse and didn't know it for a couple of hours. ciV is still much easier at the highest levels, largely b/c they don't ramp up the human player penalties at higher levels and instead just give bonuses to the ai. how much easier would deity in cIV be if you didn't get the huge happiness penalty for example? at least now we don't have to contend with war weariness...
Get rid of some of the happiness nerfs, give all AI units +25% combat strength and I'd be a lot happier.. the combat would actually be interesting, and I wouldn't be 'forced' into a linear abuse the AI's bad tactics game.
not a bad idea at all. however, lots of people would complain about that, and the patch has fixed a lot of player advantages:
1. horseman nerf is very damaging to humans b/c even post-.062 patch I rarely saw more than 1 or 2 ai horses other than UU's. -33% vs city and 10 instead of 12 combat power is a HUGE reduction in power, effectively much greater than a 25% overall bonus to the ai.
2. numerous slight tweaks to the combat system are much more damaging to the human than ai:
-flanking down to 10% instead of 15%.
-open terrain combat penalty down to -10% vs -33%. this one single change is equivalent to the theoretical ai +25% bonus in many cases b/c the ai still leaves his units sitting on flat terrain quite often.
-GG -5% for all civs and -10% for china, again almost always damaging to human but rarely affects ai
-must take combat promotions right away: no longer able to store 3 healing promotions, which the ai was never able to do anyway. HUGE net benefit to ai.
-mil tradition down to +50% instead of +100%. not as damaging now that you can't store healing promos anymore, but still makes it a bit harder to plan out promotions effective. at worst it is a wash for ai vs humans.
-huge increase in city power is probably equivalent to about +100% overall. they heal faster, horses/archers are weaker against them, they have better combat power b/c they ramp up faster due to techs now, vastly increased healing, AND you don't get terrain bonuses vs cities any more. some cities will literally be unconquerable or nearly so now, even for the best players. generally I think that this favors the human as he's much less likely to run into a runaway ai while the human will just focus on devloping better tactics to overcome the changes, but again it is at worst neutral.
overall combat is much more balanced now, and in my opinion the effective ai bonus vs vanilla is much greater than 25%. plus they don't get harrassed for adding another ai nerf. win-win!