FFXII's story at potential, but it falls flat on execution. The beginning is veyr fast paced, and does well in getting your party together, but soon after you've assembled your party the plot falls apart and you spend the rest of the game tracking through endless dungeons fighting plot-irrelavent monsters while the plot happens elsewhere. It's only very near the end that the plot starts moving again, and it leads to several plot threads either left hanging or being tied up unsatisfactorily. Many of the more interesting characters are either dropped off near the beginning (Ondore, Vossler) or only picked up at the end (Reddas, Cid, Gabranth). And the six members of your party don't make up for that.
The viewpoint character, Vaan, had potential just because of his relationship with Basch, but that's dropped and he's relegated to comic relief. Basch seems like an interesting character but that is soon squandered by the 'Evil Twin
did it' reveal and his past with Gabranth is only barely explored. Ashe, the main character, is annoying as hell and never develops as a character, despite the plot revolving around her. Balthier is the best character just by virtue of his permantently snarky attitude, but his character arc only turns up near the end and, although it contains some of the best cutscenes of the game, is wasted. His partner, Fran, is a decent character but by virtue of her character arc often seems tacked on. And Penelo? She might as well not have been in the game by how much effect she had on the plot. She literally had more lines in her epilouge voiceover than in the actual game.
FFXII's plot is about events, not characters. You spend most of the game trying to find shards of 'Nethicite' to fight the empire. This might have worked in early Final Fantasies, but it doesn't now. Most of the areas in Ivalice are beautiful but irrelevant, and the plot isn't nearly long or complex enough to hold a game of this length, so it is padded out by endless fighting.
The plot is by no means bad. It has a lot of potential, regarding the war and the Occuria
, but the war only comes up at the end (you don't even go to Rozzaria) and the main 'twist' is only mentioned near the end and not expanded on. All in all, good concept, terrible execution.