I actually like the World Congress. Yes, in VI, it's definitely odd that there's no particular input on what the high-level proposals are. That might be a rare area where I think V did something better than VI, as whoever is elected as the host in V gets to choose what is proposed.
Coming up with something that's engaging, fair, and fun is the challenge. Civ IV was the first one to introduce a general-purpose World Congress type event, and there the unfun thing was mandatory civics (and it always being the last-tier civic, which was not necessarily the best one). I think it would have been better to have a "banned civics" option - agreeing to outlaw slavery, for example. Now that you can ban luxuries, that has come under criticism... and I wonder if part of the problem is that any luxury can be banned. In my current game, Whales have been banned, which sort of makes historical sense, and the same could be said of banning Ivory. But you can also have silly things like banning Sugar or Spices or Citrus. If you knew, "I'm dependent on whales, furs, and ivory as my luxuries, I need to come up with another plan as they're likely to be banned someday," my guess is that it wouldn't feel as unfair/dogpile-y as if your luxuries are spices and dyes and they get banned.
One of the other challenges is that it's not necessarily easy to translate what actual international law represents into game mechanics. A chemical weapons ban makes sense as a proposal, but for it to make sense in game mechanic terms, it requires having chemical weapons, and that can get a little bit dark thematically. Similarly for the ban on expanding bullets, dating to the Hague Convention of 1899 - there would have to be a mechanic to represent the difference between before and after the regulation, including the potential of an outlaw nation still using them. Which... you could have that, but does it result in a better game?
Then there are things like maritime regulations. You could say all harbors produce more gold if that is passed, and I suppose that would provide an incentive for landlocked civs to vote against it, so it might work... but again there has to be a mechanic that corresponds to it, and reasons you might want to vote both ways. Civ IV has a global currency option that increased the trade routes per city, and I never had strong opinions either way on it, because it affected everyone pretty much equally. Whereas in my current Civ V game, there's a proposal for a regulation that would give 20% off tech costs for anyone behind on techs... I'm the tech leader so I'm voting against it to protect my tech lead, and have already struck a deal with Byzantium whereby they will vote against it as well. That proposal is providing proper incentives both ways.
