FFXIII wasn't a bad game, the problem is that the Serie's standard is very high
The problem is that it dropped way too much stuff people expect from FF, not that the standard is high (which it is).
There was no exploration basically the whole game, barely any weapons, cutscenes popping up after each 3 steps (they used to be special pieces).
It has a good story, and I personally liked the paradigm shift gameplay mechanic, but sadly even those two had clear weak points, the paradigm shift could be easily abused and let the game play itself and the story presentation, even filled with cutscenes, was not the best as a lot of info had to be searched instead of delivered to you.
Yes, it wasn't a bad game, but it wasn't only because the other games were better.