RFDOCIV inspired in me a love and knowledge of history, geography, politics, and researching where my local school system failed to, I still remember playing as Babylonia in high school, wondering why my capital kept getting attacked by barbarians around the same time each game, and finding a wikipedia article about a sacking of the city from that time. I have friends because that love of history and politics combined with a bizarre series of coincidences involving Spongebob, speedrunning, Youtube, Twitch, and modding, I exist because that love of history, politics, researching, those friends, and MGS2, which I probably wouldn't have played had it not been for MGRRV and memes, helped me survive my Big Shell Incident, and... well, geography didn't contribute much to my life, but I can say I'm now ashamed that, 10 years ago, if you asked me what direction New York was compared to my home state, I'd have had no clue, let alone be able to point on an unmarked map where something like China or Britain was.
Beyond being the first 4X history sim I played, I def agree with the above statements re: stability, especially with how it gives actual repurcussions for being an authoritarian or warmonger, no more whipping, drafting, and razing cities with wild abandon as your happy police keep your enemies away and your populace complacent. The UHV system is also absolutely insane! Every civ having a victory condition of their own is already tons of replayability and gameplay variety, but it also serves to fix like, every single problem every Civ game ever has had. The UHVs act as perfect tutorials, each additional civ slowly adding more elements of gameplay and testing you on each so you won't just speed past, for example, the Ancient era and not understanding the power of production hurrying. They all have wildly varying lengths, so if you only have time for a 30 minute game you have a solid half dozen civs to play, and a half dozen more if you already know their strategies. They also serve as like, actual difficulty options, instead of like "the AI all start in the Classical era", civs are harder or easier based on their goals. Not to mention that they push you out of your comfort zone and force you to experiment, I've never used a slider in a civ game as much as I have in RFDOCIV, using the culture slider for the first few turns of a new game to help cities get up to snuff is an insane strat I only just learned when playing a Japan game and realizing I'd be better off popping the BFC for the first few turns than like, 1% progress towards my next tech.
There's also the natural gameplay variety. In most 4X games, different societies usually either feel the same or only feel different because they have a single special effect or maybe a few unique builds, but here modifiers and geography work together to massively shape each civ. Some push you towards science, others towards espionage, some towards production, others towards commerce, some towards cottages, others towards specialists, and religion only further changes how you play, shaping your allies, your enemies, your production priorities, your government system and what wonders you can build, and those wonders only further shape your gameplay.
Honestly, the only things I don't like about RDDOCIV are the things that are built into CIV's engine, be it the difficulty of implementing nomadic or independent civs, which RFDOCIV has already massively improved over CIV, or the oddity of settling implying the land was uninhabited previously, or populations being largely uniform with no identity of their own, or civs spawning at fixed points instead of from separatist and migration movements.