From Wikipedia:
Mystery Dungeon: Shiren the Wanderer (風来のシレン Fushigi no Dungeon 2: Fūrai no Shiren) is a graphical roguelike by Chunsoft. It is the second videogame in the Mystery Dungeon series, and the first game in the series to feature all original characters.
As in other roguelikes, the player moves in a turn-based manner through randomly chosen static dungeons. Although the player can save the game in various locations, dying resets Shiren's level to 1, sends him back to the first town, and all items not saved in warehouses are lost. The game's dungeons include a variety of settings, like caves and forests, and are randomly filled with monsters, traps, and items.
This was released a while back, and more people need to know about it. Basically it's a roguelike, and if you don't know what that is, then it's like Diablo only turnbased and when you die, well, you really do die. Level goes back to 1 and you lose all the items you were carrying. That should tell you that this game is hard, which is already perfect for the people who complain Kingdom Hearts could be won by pressing the X button repeatedly.
If you're scared about difficult games, however, don't fret. It may be hard, but this game is designed to be replayed over and over anyway. Don't think it will be too repetitive, though. Dungeons are random like in Diablo, and deadly monsters that can legitimately kill you populate it in different corners. Items are random as well. New adventures trigger side quests that make the quest easier, like party members and a jar that can combine weapons to create uberweapons that have different qualities. Experience points in this game can be quite literal, and by that I mean as you grow in experience playing the game you can avoid stupid mistakes that lead to death and better prepare against the really scary monsters.
If you're interested, go buy the game already. If not, read this.
That should seal the deal.