Old school game design makes a more tactile experience
A lot has been said about this game's mechanics and backstory, so I'll just assume everyone reading knows the basics.
I've read a lot of Demon's Souls opinions and reviews, and they all lean heavily on the "not for the faint of heart" or "only the brave need apply" theme. There's actually a big secret about the game's difficulty; it's not that hard.
When I first picked up the game, I found myself in the category of people that found the game too difficult. I got stuck on the second level and saw my stash of souls dwindle to zero, and my supply of heal items get all used up. It's a disheartening experience to spend hours in one section and literally lose progress, and be in a worse position than you were when you started. I tried the other four worlds and ran into brick walls in all of them. So I gave up the game for a while, thinking I was done.
I picked it up again after learning what other people did to pass the areas I got stuck on. With this new knowledge, the game suddenly felt like it went from "Hard" mode to "Normal" mode. Then I found my niche, my favorite weapon and favorite strategy, and it felt like "Easy" mode.
What made all the difference was that I learned how the game's designers intended me to beat each level. It's not a straightforward game with no wrong answers. It's like Zelda; each level has an ideal method or two to pass it and your goal is to find that method. Some levels have shortcuts. Some tough enemies have blind spots or bugs to exploit. As you start to master the game's mechanics, suddenly you'll find you have more souls than you really need and by pumping them all into your character's stats, the game gets easier still.
The dangers in this game are very real. Even after you master a boss fight or a particularly hard area, you're always in danger of losing your life due to a small mistake or miscalculation. When you finally feel like you know what you're doing, the threat of death becomes your motivation and makes you play better instead of just punishing you for falling behind the difficulty curve. This is a very rewarding gameplay mechanic, but it goes awry if you happened to choose a weaker starting class (yes, some are just easier and better than others), or if you try to use a tactic that is simply not viable in your situation. If the developers put more effort into teaching new players their combat system, I doubt I would've given up the first time.
The game's level design is its best selling point. These are the kinds of levels that used to be called "dungeons." They're long, meandering paths, and as you go deeper, the monster encounters get tougher, and the trek culminates in a boss fight that, if you're not extremely careful, is pretty much guaranteed to kill you at least once.
The graphical quality is consistently top notch, although I have a couple of nitpicks. There are a few large vistas in the game, but for some reason they reveal a dark, blurry landscape instead of a sharp and detailed one. The theme of the game is a dark and obscured world, but that theme can be achieved while simultaneoulsy looking good instead of looking unfinished. Second, the areas you explore don't seem like real places, or places that had a purpose before the monsters moved in. The game's environments were lacking in backstory.
I would recommend Demon's Souls to players that like methodically exploring and conquering new areas, and even re-exploring them to find any hidden items they might have missed, while I'd say that players who demand cohesive storylines or prefer traditionally balanced RPGs can afford to pass it up.