Okay, finally get to write about this...
I liked Civ 5's WC for its flavor, simplicity, and transparency. You meet everyone in the game, which opens the international discussion table, and you get clear "Yay or Nay" proposals. You know how the options are pitted against each other, and you know that you are voting for your own real benefit. You could also negotiate with AI to vote for a particular outcome in the diplomacy screen.
The problem lied in AI's voting for luxury bans non-stop, and occasional stupid proposals slipping through and taking effect forever unless repealed (good luck with that). You could say that many proposals were just not thought through and turned out to be rather dull when put in practice. The fact that there was always a host making proposals meant that many times the proposals had little relevance to the participants - especially if the host is AI.
But at its core, the concept was flavorful, was easy to follow, and clearly signaled the midpoint in your game. Which is why I think Civ 5 WC had a good underlying concept, but it fell short on resolutions and how AI handled it.
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I will remove Diplomatic Victory considerations from both versions, because they are both equally bad in my book. Civ 5's strategy of saving Gold and then spending it all to bribe CS and gain game-winning votes is not that different from Civ 6's strategy of taking over CS, saving Favor, and then spending it all on the game-winning votes. Using a unique currency for the votes doesn't suddenly make the victory different and innovative, when the underlying principles remain the same.
There is, however, one DipV-related nuance that makes Civ 5's verson of WC MUCH better in my opinion. The sessions become more frequent in future eras, starting from every 30 turns in Renaissance and getting as fast as every 10 turns (20, depending on how you count the DipV sessions) in Atomic+. This means that the World Congress becomes a more active factor in the late game, making you engage more often in the otherwise barren late game. This also speeds up the Diplomatic Victory, giving you a clear sense that it is time to wrap up the game. Civ 6's fixed session frequency becomes almost unbearable in the late game, when you have almost nothing else to do and most likely could have already won via any other victory condition.
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Civ 6 WC tries to fix the issues of its predecessor, and in some ways it does. You are no longer stuck with endless luxury-banning proposals, and no accidental silly resolutions last forever. The proposals themselves are set in a way that something is guaranteed to happen after the vote - no longer can you keep downvoting the resolutions and prevent anything from happening. Finally, the Favor cost and refund mechanics mean that you actually need to budget your voting power,even if you don't plan to pursue the Diplomatic Victory. You can spend a lot of Favor on a few specific resolutions that may heavily affect you, but that leaves you less powerful for other resolutions - as you have spent your Favor, while the opposing side got it refunded. This does feel like a system that is used very effectively...
...but why does it have to be so convoluted and slow-paced? The current system slams two separate voting phases into one, creating an extra mental barrier when voting. What often ends up happening is that you inadvertently help your opponents. This scenario was illustrated multiple times here in various threads, so I will just make a brief example: If I want to get more amenities from my duplicate luxury resources because I got screwed by RNG, but I also know that Scotland wants to supercharge his UA by enhancing his own duplicate luxuries, then I am actually discouraged from voting for my preferred option. Because my votes will be counted in favor of Scotland as part of the first round, and then he may actually win if he invested more votes into his resolution. This would never happen under the Civ 5 setting - you voted for your own benefit, and nobody gets to take advantage of your votes while voting for something drastically different. Some people say that this is for the better, that this makes you think ahead. I am sorry, but I call BS on that. This is not creating a meaningful brain puzzle, this is making the votes unnecessarily complex by obscuring transparency. Yes, this is kind of a Prisoner's Dilemma. No, there is no place for this in the World Congress, where AI votes selfishly without such considerations.
The best part is - the game already has the infrastructure for better handling of the two-stage voting. Have you noticed how emergencies work? If you are eligible for emergency proposal, you can spend 30 Favor and bring the emergency to the Congress floor. Then the players get to vote for or against this emergency using the same WC overlay. This can be totally adapted for regular resolutions. Let the game present you with the two opposing options: for example, the one about amenities for the duplicate luxes or no amenities from the lux. Every player selects their preferred first-stage resolution - for free, instead of the 30 Favor. The winning option gets passed to the voting floor, and there everyone gets to pick the lux they want affected by the resolution and dump as much Favor as they want, just like now.
This post is getting way too long, so I will just wrap it up with this: Having half of the world as question marks on the voting floor and calling it "World Congress" is ridiculous. Having the fixed session frequency without acceleration in the late game is a sure way to make people forget about the existence of Diplomatic Victory a month after the expansion is released.
TL;DR: If we could take Civ 5's simple concept and incorporate Civ 6's proposal system while also fixing the current two-stage voting, this would be golden. For now Civ 6's WC fixes some main issues from Civ 5 while breaking the stuff that Civ 5 did right.