Its not the "difficulty" per say thats hardcore about it, its the concequiences of dieing that makes it hardcore.
Because you lose all your "money" (souls) if you die, and have to return to the beginning of the stage, it makes dieing a REALLY big deal, there are no save points, checkpoints, or anyway to "save" the game, although it autosaves all the time, so you can leave the game and pick up where you left off.
Also, there is no way to pause the game, the game continues to run, even when you are in the menu / inventory / equipment screens
As such, its seriously tense and foreboding, you really, really have to think tactically about each fight, and the sound effects and graphics just really pile on the atmosphere.
The "multiplayer" side of it is really well done and unique.
All in all well worth the price of importing it, I got a Chinese/ENG version off ebay for under £40.