Not all that much, actually... it was just the first other world, well city from another world, I heard about in Luceti. Republic City's not a big as it, I think. But it's a big city - the largest [with possibly exception of Ba Sing Se]and most modernized one in the world. The four nations all have representatives to a council that runs the city's government, but for the most part it's treated independent of them - and because it's the most modernized city, it's a haven for industrialists and new ideas of every sort.