I believe in the power of humanity.
I don't know how related this is to the topic but eh.
It bugs me that so many people are hostile towards XII because of how different it is from the more popular entries like VII, VIII, or X. And while I used to believe it was utter crap, XII is probably the last game in the series worthy of the label. To me, FFXII continued a logical and welcome evolution in the franchise, with much improved writing, an excellent and open-ended character advancement system, and a unique story that relied less of melodrama and more on subtlety and narrative. In many ways, the jump from X to XII is not unlike the jump from VI to VII; while many people may cry afoul, it truly is an excellent and worthy addition to the franchise. I truly do hope that Square-Enix continues in this direction in the future.
I think one of the things I liked most about FFXII was the way the soundtrack and locations seemed to go together seamlessly. Phon Coast and the Cerobi Steppe have already been mentioned, but I'd also add Sochen Cave Palace and To the Place of Gods as other pieces that stood out to me. I thought the game blended the visual and atmospheric elements together so well to create one of the most engaging and impressive environments yet.
I also quite liked the battle system. Initially I didn't, but I think it suited the theme of the game well. I liked actually exploring the environments, something that I don't really think was present in previous games in the series which were much more linear, and didn't really present much opportunity for actual exploration. Either way, the battle system made doing so a much more pleasant experience. One of the things I hated about random encounters was being about three steps away from the next town on the world map. In FFXII, I could either avoid that battle or it could be over with minimal disruption to me playing the game. I also think the battle system led itself more to encouraging you to think both strategically and tactically (i.e. long and short term) in terms of your battle plans through the gambit system, whereas previously games made you focus more on just the battle in front of you.
Other things that stood out less obviously to me (but are still definite strengths) have been mentioned by others already: the quality of the script and voice acting, the world of Ivalice itself, and the good storyline. I think FFXII is probably the most underrated game in the series, or certainly from my experience. It didn't seem to leave a mark in the same way that VI, VII or X did, but I don't think that's necessarily a reflection on the quality of the game itself.
I've taken to simply disregarding the opinions of people who dislike FFXII, because they're obviously stupid.