Take a deep breath.
Demon's Souls has gotten a lot of attention for being a very hard game. This is one of the most intense games I've played in a long time, and is definitely a challenge. That doesn't mean, however, that this game is overly frustrating or impossible at all. There were only one or two points in my near 40 hour play through where I got frustrated enough to stop playing for a little bit, and I'm sure many who played the game got through it without many worries at all.
This game really just is what it is - there is only one difficulty, the enemies don't scale to you, and once you beat the first short level the entire game is opened to you. This is an interesting design, but the way the game throws you into this perilous world with little to no instruction will probably make some people feel lost or overwhelmed - I know I felt that way. That might be my only complaint about the game, but then again, the fact that you have to literally figure everything out on your own makes it more rewarding once you do, although I'm not sure that offsets the initial frustration.
Regardless of how you approach it, the world of Demon's Souls is intricate, mysterious, and famously dangerous. You will battle a well put together array of enemies across five distinct and interesting worlds, wielding all sorts of weapons and spells. Each world is subdivided into smaller levels, at the end of which you will face a unique and (usually) terrifying boss. Some of the bosses are pushovers, but a good number of them are interesting, challenging, and very fun to fight.
The way you improve your character is another aspect of Demon's Souls that serves to separate this game from others. You have 8 stats that you can increase one point at a time in whatever order you want. Your class determines your starting stats and equipment, but once you start the game, you can literally mold your character however you want. To upgrade a stat, you use the souls you gain from killing enemies, which can also be used to buy items, and repair or upgrade equipment. Souls are literally everything in this game. If you die, however, you will temporarily lose all the souls you haven't yet spent, and can only regain them by reaching the bloodstain that appears where you died. If you die again before you retrieve you lost souls, they are lost forever. This may sound harsh, but I really think it is a smart decision that serves to create the feeling of intensity that really drives this game. It's worth noting that this is an RPG with plenty of good spots for grinding, so losing souls (the equivalent of experience) is never a game ending mistake.
Another innovative aspect of Demon's Souls that deserves mentioning is the way it approaches online play. If you so desire, you will be constantly connected to the server. At any point, any player can leave a message on the ground, potentially warning of an enemy ambush ahead, or revealing a boss's weakness. At first, I expected to get a ton of fake messages, warning of enemies in safe areas, or telling you to use a bow when you really need a sword, but I was pleasantly surprised that people who wrote messages were generally helpful. Other ways you can interact with other players is by leaving a sign where other players can recruit you into their game to help in a part they are having trouble with, and you can also "invade" another player's game to attack them. There are benefits to doing these, but I never tried either in my play through, and also was never attacked as I played.
There are many things I could go on about with Demon's Souls, but I think the highlights are the interesting world, the deep character progression, and various smart design decisions that give the game a distinctive intense feel as well as challenging, enjoyable gameplay. This game is a success no matter how you look at it.