Demon's Souls is really a one-of-a-kind game in this generation (I'm aware of its predecessors on PS1). And the difficulty has been blown way out of proportions, as I don't find it difficult in the traditional sense. This is a very fair game, if you die, it's your own fault. And believe me, you will die. Play it safe, and if you don't play it like an ordinary hack'n'slash, it's not that bad.
Also, you will spend most of the game with 75% health - during the first level, you will spend most of the time with 50% health. That's because if you die, you restart in soul form with 50% health. During the first level, you get a Cling Ring, which means you'll be able to have 75% health in soul form.
Also, the currency used in this game for buying stuff and upgrading your stats and weapons are souls. Whenever you kill an enemy you gain souls, like when you gain money to kill enemies in other games. If you die before spending them, you lose them and start the level from the beginning. To regain your lost souls, you need to get up to the point where you died, and interact with the blood on the ground. However, the game only lets you pick up the souls from the last time you died, so if you die again when trying to retrieve your souls, those souls will be lost forever.
Thankfully, you can leave the world (level) whenever you want, so it's possible to go for a soul hunt, then leave when you have enough.
Oh, and there is no pause in this game. So make sure you're at a safe spot when you go for a pee. Don't worry, there are planty of safe spots. Once you've cleared out an area/room, it's completely safe. Enemies don't respawn until you die. Of course, there's always the chance of getting invaded by another player though...
