I can't understand why nobody mentionned the roguelike.
Diablo was not an original game, it was a glorified realtime roguelike. Rogue is the first action-rpg game, at a time (1980) where every game was just using text, rogue was using a top-down view with ascii art. Then came Hack and NetHack, which is the most played roguelike right now.
And guess what, Nethack has been ported to the nintendo ds !
Beware these games are a lot more complex than Diablo, first you only have hardcore mode, so one death and hop, then all the identify scrolls of Diablo comes from here, instead that in Nethack these scrolls are hard to find, so often you have to guess, you wan't to know if an object is bless or cursed, put it on a altar and it will glow strangely to hint you, you are almost dying and you have a bunch of unidentified potions, which one to drink ? (paralysis potions are really unfriendly in this case)... Add to these that the average playing time for hoping finishing this game is between one and three years, and you have a winner. Forget Diablo !