Man, I still need to finish this. I'm on the fourth day with Beat if I remember correctly.
But yeah, it really is an amazing game. They had so many interesting ideas, it's hard to list them all. The difficulty slider was one of the most genius ideas in my opinion though, it allows both veteran and novice gamers to enjoy the game and the story in its entirety. For those who don't know, the difficulty slider is basically a line that you can slide to the left or right, starting at one and going up to whatever level your character is. Doing this will either increase, or decrease your HP. Those who choose to play on the edge with very low HP will be rewarded with more items when they defeat enemies, while those who do not can still enjoy themselves without wanting to break their DS in half.
One of the best on DS in my opinion, everything about it was just so unique. Anybody who has a DS deserves to give this game a chance.