I think we will see more and more "open ended" worlds in RPGs as the hardware power increases.I don't think developers of previous FF's opted to go for linear environments because they wanted to,but because of the hardware limitations.FFX and FFXII may share the same console but bear in mind that FFX was one of the first games developed for PS2 and it's developers didn't have the technical knowledge that was available near the end of the system's "lifetime".I'm pretty sure that both FFXIII's will feel "open ended" but it's too early to speculate on that.

I also loved the environments and locations of Ivalice,there were times that i felt really immersed in that world.The music,although at first i didn't like it,really grow on me and now FFXII's ost is one of my favourite FF soundtracks.When i played Revenant Wings i really enjoyed hearing some of the FFXII's themes...In general i think the world of Ivalice was one of the most vivid RPG worlds to play on a console and it really felt "alive".Also,most of the NPC's had some really interesting stories to tell and they really complemented their surroundings (the Old and New Archades is the first that comes to mind).
Unfortunately,as Roogle said,these will be my only main reasons to replay the game (including gameplay/excluding level grinding) and not it's characters or story.