You can't really say much about Vaan because the most dialogue he had was in the very beginning aside from that he barely interacted with the rest of the cast basically almost being non-existent which is what pissed me off about 12.
This. FFXII's script was bare-bones. The character development was non existent, and for a Final Fantasy game that is a critical error. Even among die hard FFXII fans I have debated this with there has been an acceptance that the story itself was completely lacking. The voice acting might have been pretty good (at least Balthier), but the sound quality was poor, and the script was "Hey, we need to get to B, so let's get there! YAY!" The bosses in the game had nothing to do with the plot, because there wasn't one. You'd be walking along when all of a sudden a big tree lizard attacked you for no reason other than "it's a boss" (I really thought we'd moved away from that sort of thing).
I don't want to get into a big debate about this but I guess that's what happens ;D The game-play was lacklustre too imho. Gambits took all the tactics and brainpower away from you; for 90% of the game I survived on 2 gambits as my characters mindlessly slashed away. Random treasure was a silly idea. The areas were great but filled with nothing as you walked for miles on end, and sidequests and user input was down to a minimum.
In FFX you get complete control of your characters and even Summons, you get little quirks with the limit breaks (like entering a code, or pressing button at correct time, or liming up slots). There is nothing like this in XII. It's just one long trudge, and the gamer comes second at all times. Even the AI was lazy. FFXII deploys the most lazy battle mechanic too "Heaps of HP = harder".
It's a complete mystery to me how anyone puts this game above X... I've racked my brain over it a lot, but I haven't got an answer!

The only thing it beats X on imho is the graphics (and perhaps voice acting, which doesn't really count as part of a game anyway). Even the music has to go to X.