If we go by this, the entire Harry Potter series sucks.
I'll leave my personal opinion of Harry Potter out of this
Though I haven't red them, I have seen snipets of the movies and it seems to me like it is done in reasonable fasion. The story acknowledges that there are questions that haven't been answered, and they will have to be answered in the sequel.

In FFXIII there is no such acknowledgement. The game essentially wraps itself up in a neat little package for a chliche happily ever after ending. Had it even been brought up in game it would have been an element that we have to wait to find out about. As it is it looks an awfully lot like they just didn't take the care to fully think about their plot.

I promise you, this game has "just didn't care" plastered all over it
That is where this is coming from. I spent the whole game going through hopping that the plot that made no sense would be wrapped up somehow in the end. This didn't happen. To me it looks like the writing team was so caught up in their character interactions that they whole plot was conceived just to get the characters into situations to exchange the dialog they wanted them to say. If they want to focus on characters and have a simple plot they should by all means go for it. Not my cup of tea, but I can deal. Just keep the plot simple and making sense.

EDIT To Arcane's edit: I started this thread hoping someone could explain something I missed so I could actually accept this games plot. I would much rather like a game than dislike it. Doesn't look like that is going to happen