I found it to be the first Final Fantasy that I truly disliked, for a couple of reasons.
Firstly, the characters themselves. Vaan annoyed me no end, and while the graphics were good, the way he's built is just wrong for a human. As in, no human will ever look like Vaan, in all time, ever, despite how good the graphics are.
Secondly, the character development. Final Fantasy 7, you added to your characters with materia. FFVIII, with Junctions, etc etc. FFXII, you unlocked your character slowly. Thats a crucial difference in my opinion- the previous systems allowed you to customise ways of making your character more powerful, while the new system was more to the lines of a way to bring your character up to par. It's difficult to explain the difference, but it's an important difference.
Thirdly, the story had potential, but I honestly couldn't bring myself to care about it. Oh no, you're country was attacked, and your brother died. Life goes on. The guards are as*holes, but it's not like they're killing the planet or making your life particularly bad. You have food, shelter, friends, a past, a present and a future. So does everyone you know.
At some point I might give it another go, and I did think the bounty hunting system was pretty cool, but compared to being able to play C&C 3, it doesn't seem appealing.