An action-RPG that prides itself on its difficulty, Demon's Souls is not a game for the faint-hearted. However, for a player willing to overcome a few hardships, Demon's Souls is a well-made, well-executed, highly enjoyable game.
Demon's Souls' basic gameplay is hack-and-slash similar to The Legend of Zelda or Kingdom Hearts, i.e. you swing your sword, block with your shield, roll, dodge, and so on. However, Demon's Souls has much higher stakes - your character can die in 2 or 3 hits, for one. Enemies telegraph attacks well enough, and are generally as weak as you are in health terms, so it's at least fair in that regard. It's less that the game is hard directly, and more that the game is unforgiving of carelessness; charging at an enemy as though they can be easily dispatched, like you would in most action games, usually ends up with your death.
There's a fairly wide level of customization available in terms of gameplay choices, though. Souls are the game's currency, and are obtained from every enemy defeated. They are used to buy stat upgrades, spells, weapons, armor, and items. In essence, you have a fairly wide set of choices available in terms of how you play the game - as a direct fighter, as a lightly-equipped archer, or as a supporting magic-user. Your class influences your starting stats, but there are very few class-based limitations in terms of equipment and spells. As long as you've upgraded your stats to the right level, you can use almost anything.
There is some difference in terms of what weapons are used, so there's at least a lot of diversity in terms of your character's development. In addition to enemy weaknesses, the weapons in the game also take how the weapon is used into account. Most directly, walls and corridors will make it impossible to swing a weapon - a problem if you're using a giant sword and can't get the room necessary to swing it. Generally, stabbing weapons like spears and rapiers are useful in close-range areas, while the giant swings of greatswords and polearms are useful in wide-open areas. It's possible to wield weapons one-handed or two-handed; it's also possible to dual wield weapons, to varying levels of effectiveness. The two kinds of ranged weapon are bows and crossbows: the former can be aimed manually, but requires two hands, while the latter must be locked on automatically, but can be used with a shield.
Death in Demon's Souls is frequent, and comes with a few penalties. Upon death you become a ghost, with only 50% of your normal HP (though this is raised to 75% by an item found early on, which is much more tolerable). When you die, you lose all your current souls (but not items or abilities), but if you can get back to the spot you died in and touch your body, you can at least recover it. There are a few ways to recover your body, the most common of which is beating a boss while in spirit form. However, it's often not worth it - you can only beat a given boss once, and it's fairly easy to die afterwards to some simple trap or surprise attack. And given the number of things that are trying to kill you - soldiers, monsters, dragons, rolling balls, pit traps, and the occasional explosion - care should be taken at basically every opportunity.
The game's multiplayer is probably its most innovative feature; rather than a direct sort of "join someone else's game" feature, Demon's Souls is more like a persistent online world where you can only occasionally interact with other people. The most common online interaction is messages. These messages are assembled from premade parts ("look out for the ____ ahead", etc.) and left on the ground near whatever they're talking about. Any player can put down a message, regardless of content, so it can be used to help people ("there's a trap ahead", "don't trust this guy") or to hinder them ("if you jump down into this bottomless pit you'll get some treasure"). Trustworthy messages can be recommended; recommending a message provides the player who put it down with an instant full-heal - something that's not to be sneezed at, especially during boss fights. Therefore, it's to the player's advantage to put down helpful notes, and the game is certainly hard enough to warrant them. In one case, for example, an otherwise innocuous NPC was marked with many runes nearby indicating that said NPC was a liar and I should attack him. As it turned out, the NPC was powering a ritual that made the level's boss immortal, and without killing him I never would have been able to beat said boss.
The other way that the game presents a passively online universe is through player ghosts and bloodstains. The former is just a glimpse of other people playing the game - i.e., their characters running around in the same level you are in. These are mostly just there for show, though seeing a ghost pull a lever or break down a wall can provide a helpful hint. When another player dies, their bloodstain is left in the location where they died. If you touch a bloodstain, a ghost pops up and runs through the last few seconds of that player's life. This basically shows you where traps are in a lot of circumstances - you just have to watch for the part where the ghost "dies" when you go forward. In some cases, the presence of a great deal of bloodstains serves as warning enough.
There are more direct ways of interacting, as well. When you're a ghost, you can offer your services as a Blue Phantom and enter another player's world as a cooperative helper. Beating the level's boss transports you back to your own game and gives you your body back. You can also take the darker path of the Black Phantom and invade another player's world; if you manage to kill them, you get your body back. Overall, there's not a lot of communication possible in-game. The messages serve as the only actual communication; blue phantoms must communicate with gestures and emotes, and there are only a few of these. There's no friend lists or anything along those lines, so you're really just grabbing any random player either as an enemy or a friend.
The graphics in the game are excellent - the designs and effects are detailed and well-executed, there are plenty of incredible panoramic views (many of which can be noted with messages) and the characters all move with realistic weight and effort. On the other hand, the Havok physics used in the game are completely ridiculous. Bodies, for one, can be easily kicked around by a player walking slowly into them, which causes them to go into humongous spasms and flail around. The sound is great, being distinctly atmospheric while also serving as a warning. The music is good, but only shows up for boss fights (in a manner similar to Shadow of the Colossus), while the rest of the game is music-less to let you focus on approaching footsteps or flapping dragon wings.
As a whole, Demon's Souls is a great game with solid gameplay and fun online multiplayer. However, it's also a very unique game - casual gamers, people who dislike losing repeatedly, and people who want to play with people they know in real life are probably better staying away. The game's difficulty will drive away most gamers; even gamers who stick with it will likely find it frustrating when they die for the hundredth time. The online mode, while interesting and fun, is limited in its long-term applications due to the short "pick up game" nature of coop play.
In conclusion, the game is a 9/10 for hardcore gamers, and a 6/10 for casual gamers.