It's hard to explain. I actually really like the graphics, though they have been described by some to be less than stellar. My only complains are the frame dips and lack of shadows. There's no screen tearing, at least, which is awesome. The music is also a nice fit for the game, and is only there when it needs to be.
The voice acting is great. It really fits the characters, though they're can be some strange pauses between words occasionally.
There isn't much story, but it does what it needs to do, and the NPCs, given their small numbers, feel more fleshed out than those in a game like Oblivion. Usually when I say "does what it needs to do". that's not really a good thing, but the story really works with Demon's Souls, for what it is.
To be honest, I love the game. I'm not sure why, because it many ways it seems like a game I'd hate. I've always had trouble getting into more Western RPGs, and I usually give up on tough games after just a few hours. I'm over 30 hours into Demon's Souls, however, adn still going strong.
Tbh, I think what I love about the game is the combat and the general setting. The combat is extremely simple, yet quite deep. You really have to learn how to use the combat system well in any situation to survive. Your character also has a rather heavy feel to him, and the animations are nice. I think I love they way they've really tried to capture the feel of a knight roaming around in a medievil-ish setting, and they've done the job superbly. I also love how it's an RPG that doesn't really bog you down with a bunch of numbers and stats. Yeah, you can upgrade your weapons and increase your stat points, but just as crucial as either of those things is learning the combat, learning your enemies, and just playing smart.
I'm more of a casual RPG gamer, but I love this game.
Overall, I'd tie it with Uncharted as my third favorite ps3 game. That may change over time, as I get deeper into the game, and as my opinion has time to grow and deepen.
I'd say my list of preferred ps3 titles looks like this, at the moment:
LittleBigPlanet
Metal Gear Solid 4
Uncharted
Demon's Souls
Killzone 2