I haven't beaten the game yet, so feel free to look contemptuosly upon my comments if they disagree with you :)
I think that soccerdrew17 expressed the feeling I've gotten from the game so far pretty well (forty hours in). It is a great and solid game, but there's something missing which FFX definitely had (X also had some annoying voice actors and XII avoided that.)
What's missing is hard to describe, because it IS a great, well-polished game...
There's no leaving Midgar and realizing there's a whole wilderness of a world to explore. There's no creepy realization that you have to use the lives of an endangered race (your own race) against the empire who is preying on them in order to bring about justice, and then failing in your task only to realize there's a whole new world to explore and save. There's no slowly unfolding mystery where it turns out the whole world's drama is tied up in your existence. The Final Fantasy games do a uniquely good job of giving you this expansive feeling toward the world that you're exploring, where they pull back the curtain one step at a time and the world gets both wider in scope and deeper in meaning as the game goes on. XII hasn't really given me this feeling -- most of the cards seem to have been already on the table at the beginning of the game. I'm hoping the ending changes my opinion, but I'm far enough through now that I think I can understand some of the complaints.
I agree that the combat system works really well and is a good new realization of the Final Fantasy ATB system. The one problem with it is that while it feels really new for the FF series, it feels really familiar (often in a bad way) for anyone who has played an MMORPG in the past decade. Yes gambits are new, but they're introduced into the game too gradually for something that is supposed to make the game somehow more tactical than past ATB systems. A combat system that really looks like it's trying to do something no game has done before is more exciting than one mostly borrowed from a neighboring genre. However, I'm glad to see its not a permanent direction for the series, and as an adventurous departure from the old systems I think it was a success -- I thought they made some of the battles in XII a lot more challenging than they had in other recent games, without making them impossible for beginners. It's definitely a fun game, even if they overdid some of the respawn in areas. (Exploring sometimes feels like playing an MMO when no one else is online, which gets repetitive fast.)
The story is well written in terms of the actual dialog, and the voice acting is excellent, but the storytelling doesn't move quickly enough -- the pacing is off. That the lead designer of the game wanted to make the main character Balthier and was told there had to be a younger lead that teens could relate to makes me think maybe too many cooks spoiled the soup. The story I've heard is, said lead designer then had some sort of nervous breakdown and quit working on the game partway through. This suggests to me that story could have been very different without all those development problems.
I really want to love FF XII, by the way. Since I got a PS2 I've played through every Final Fantasy game since IV with my girlfriend (a lot of them are 2 player in battles). It's one of the only series she likes so I'm always hoping they can outdo themselves and make an incredible game. FFXII is interesting, almost like a Final Fantasy for grownups, but with a pup of a main character. There just aren't many images or feelings from the game which stuck with me since we got a little bogged down in it six months ago and started playing Tales of Vesperia instead. And in the end, that's what I want the most from a Final Fantasy -- those lasting impressions and images from the worlds and characters they create.