The latest Game Developer Magazine has a full postmortem of Final Fantasy XIII, featuring Square Enix developers Motomu Toriyama and Akihiko Maeda revealing just what went right and wrong with the thirteenth iteration of the classic series. The main issue with the game, they say, was a lack of "shared vision" among the developers for what the title should be. Some developers wanted to create a game to answer Western concerns about JRPGs, while others wanted it to serve as a showpiece for a crossplatform engine, and still others wanted to simply emulate the feeling of that first trailer back in 2006.
In the end, what made the vision concrete was the creation of the demo available with Advent Children -- that demo changed the team's talks "from theoretical discussions based solely on abstract concepts to concrete dialogue," and helped the team realize the kind of work they'd have to do on the title before release.
The few clips of the postmortem on Gamasutra don't mention the linear aspects of the game that turned quite a few players off, but the devs say that the more realistic picture of the production brought about by that demo helped them target what to work on by "keeping in mind exactly how the asset would be used in the game." In other words, focusing on one path was supposed to help them make sure that path was worth playing. But of course it's up to players to decide how that worked out.