While Final Fantasy XIII was a solid game, one major problem that fans had with it was its linearity. For the first 20-30 hours of the game, you're stuck on rails going from point A to point B, killing everything in your path, watching a cutscene, and then repeating the process again.
We already know that Final Fantasy XIII-2 is going to be a more open-ended, traditional-feeling RPG, and a part of that feel comes from its Time Travelling mechanic. By finding special items called Artefacts, you'll be able to use them to open up different time periods for you to explore the same area--often with big changes in each area that have happened over time.
Square-Enix is really pushing the idea of actually getting out there and seeing everything that this game's world has to offer. That's pretty cool--I'm looking forward to digging in when the game hits on January 31st. What about you?