Yeah, I was similarly down on Xillia. I don't know if I was just burnt out on Tails games or what, but I couldn't ever find the motivation to finish it. But I loved Graces F...
Anyway, speaking of finishing stuff I did finally finish this game. I enjoyed it overall, but I definitely have many criticisms. There was an overall lack of polish to the whole thing...I feel like the ideas were solid but it really could have benefited from a longer development cycle. I mentioned the verbosity and pitifully short dungeons already and that much stayed true 'til the end. In fact there were a number of places in the latter third of the game where I was really expecting a boss fight but the game decided to just handle everything in completely non-interactive cut scenes...kind of lame.
The combat system had potential but I feel like they just didn't do enough with it. In addition to the short dungeons there was a ridiculous amount of enemy reskinning. Everywhere you go you fight the same old shit just with slightly different color schemes. Heck, even
when you travel to Ar Ciel, the Ar Tonelico world, which is a totally different world than where most of Ar Nosurge takes place you're fighting the same old enemies. That doesn't even make sense. Plus every song is just attack magic and the only thing that really distinguishes them is that later songs are...well, better attack magic. They throw in this system where you can't use a particular song for an encounter if you've been using it too much and that's literally the only reason not to just always use to latest song you got. It feels artificial.
The wave mechanic is kind of neat at first but becomes a bit tiring in the sense that the most logical move is always to build up your song to the point where you can wipe out all the enemies in a given area. So you fight one sorta-long battle per area instead of a lot of little ones. Is that better? I don't know. The thing is I was just always using the same attacks to get break after break after break until I had built the song magic up enough to kill everything. Every battle, every time. The game desperately needed more variety.
Boss fights were the only thing that broke this pattern, since you couldn't just get breaks every turn and actually had to defend yourself. But they also felt a little bit cheap at times. See, unlike AT2 in the defense segments you don't have a gauge or anything telling you went to block, you're supposed to just figure out by watching the attack animation. But some of these boss attacks had extremely elaborate animations and it's not really clear when you're supposed to block if it's your first time seeing it. On top of that you are only allowed a limited number of block attempts per turn so if you jump the gun you can burn them up before the attack animation is actually finished...
OK, enough criticizing. Things it did well: Music, of course. The story was interesting, and the characters were pretty good for the most part (though there were a few exceptions...). Heck, even though they started out as a pretty standard clueless hero and tsundere tropes I actually felt that Delta and Casty's relationship eventually developed into one of the more convincing video game romances I can recall. The dive segments were also handled pretty well. I enjoy seeing the darker, repressed side of characters like this. The more...meta aspects of the story were certainly novel. I don't think I would want to play a lot of games like that but it's a nice change of pace once in a while. The combat system had its enjoyable parts too, it just desperately needed more variety.
In the end I feel like in some ways the game's biggest problem was that it was just too ambitious for the budget and resources they had to make it.