The battle theme. I didn't like anything else, especially with the way the game tried to go out of its way to tell us what the story was.

Imagine you're in a cinema, watching a film, and every five minutes the film pauses and a giant caption appears saying TURN TO PAGE 6 OF THE ENCLOSED BOOKLET TO FIND OUT WHAT THESE CHARACTERS ARE ACTUALLY TALKING ABOUT.

That is what FFXIII did. I found it so obnoxious.