This one is less easy to diagnose. The battle system has many nice concepts, and can be quite enjoyable, yet at the same time can become cumbersome when you're fighting weaker enemies. The purge system is a great way to try and add spice to the battles, but unless you're really devoted, you likely won't take advantage of the way using rhythm can help you get your purge ready much faster. I didn't really find this out until my 5th or so playthrough. Before then, I mostly just button mashed when the bars were higher. I also learned that the hyuuma you have equipped has a great deal of influence on how the bars work, because it changes the song. There's quite a bit of depth to be found, but at the same time it's unnecessary in the end is the problem. Outside of combat, you have the always helpful Ar tonelico encounter gauge.
This helps to alleviate the frustration of random battles. How it works, is you have a bar that starts out full. When you finish a battle, the bar depletes based on various things about the battle you just fought. When it's completely depleted, you can run around freely without any fear of being attacked by a random encounter. This is nice for collectors who are scratching their heads trying to find a treasure chest. Very useful, and I'm sure everyone's wishing that something like this would have been available in the RPG's of yore with random encounters. Last thing to talk about would probably be how exploration works out.
Exploration is one of two things: travelling on foot, or using the world map to QT between places. You can walk to any place on foot if you really want to, but there's still a quick travel function if you want to get somewhere quick and without worrying about random encounters. The QT system was also implemented well, because at certain points in the story when it would make no sense for you to actually go to some places you've been before, it won't let you go those places. Now, while exploring, you also can run across treasures, or talk topics. Talk topics are what are needed to be able to dive further in a reyvateil. Diving is basically how you get to know a reyvateil partner better, and a large part of the Ar tonelico experience. So, these talk topics are important to look for. And then there's the synthesis system, and I think that covers the gameplay in a nutshell. there's probably a lot more that could be said, but I've already dragged on this section long enough.