IMO, when it was released at the time, it had the unique factor of being an ARPG, something which the DS system was lacking at that time.
Plus there were the beautiful graphics, smooth controls and was incredibly gamer friendly, auto mapping, resettable skill trees, and not to mention multiplayer.
Even now, i would consider its multiplayer system to be superior to that of Kingdom hearts 358/2 days and I guess its the only selling point at the moment, no other ARPG comes close to it in this aspect.