I love how you always have to say its an awesome game after every criticism you make of it, like you're trying to convince yourself its not as awful as you subconsciously know it to be.
I also agree with Flying Arrow, while progressing the story is often linear, the world itself grows and expands as you progress and your options get greater. It creates a greater sense of a world, than just traveling from one connected location to another, and the tube like quality of these stages only help to make them feel artificial. Some dungeons in previous games were also simply straight paths, but the games tended to scramble it all into a twisted mess, so its hard to tell how linear areas are when they resembled mazes. Mt Nibel itself is like five maps top, but it involved hitting switches and solving a puzzle to get to the boss and the exit. IV had several hidden paths and some large branching areas in their dungeons which made their visually simple designs far more complex. They may seem simple but really when you think about it, even some of the more simple dungeons in early FF games tend to be less straightforward than what XIII presented us.
I think one of the bigger issues with some of the later FFs is how sparse they are about vehicles. While both X and XII had a form of an airship, they are revealed at the last stretch of the game when the final dungeon opens and you're in sidequest hell. Even FFVIII and IX were really sparse about giving the player the ability to travel around the world with the same sense of freedom as an airship can. I feel vehicles also make a world seem larger and bigger, and by the PS2 generations, they have been marginalized to a great degree.