We discussed barriers, geographic, cultural, linguistic, technological, with both transport and communications. One audience member described a world that had unified into an empire because it was mostly land, and we tore that to shreds: if you look at the photos of Earth from space taken at night, you'd think humanity was amphibious, and travel by ship, with all its nightmares, has been easier than travel by land -- which could have some formidable obstacles on its own.
Cultural diversity. One person thought Star Trek had actually once managed to put two warring cultures on one planet, but was corrected: two planets. I brought up John C. Wright's The Golden Age trilogy where you do have a single government, very limited, but widely different cultures that need Sophotechs -- AIs -- to communicate, and a College of Horators does most of the actual social control.
Governments range as far as they can control. Which is where communications technology really gets into play.
Balkanization on earth and whether we would export it. The difficulties of FTL: if it's easy, we might go to the choice spots and only spread out slowly even there. If not, we throw everything we can. How often the new ships can arrive would mean a lot, especially without communication. And if the government switched in the meantime. . . . I brought up Rick Cook's Limbo System because the aliens there live in space and so expand slowly. Indeed humanity has a FTL drive that the aliens don't because we really wanted to get out there and find planets.