I don't really know where to start, as there are so many good things about Dead Space.
GRAPHICS........Dark, moody and great animation of the necromorphs. But the best thing about the graphics is the lighting, dark areas with a flickering light, casting very disturbing shadows, only to realise its really nothing at all.
AUDIO.......For me the best part of Dead Space was the sound. Very realistic alien screams, whether they be right in front of you or off in some distant part of the ship. Its the random noises that keep you on the edge of your seat though. Just never knowing if the noise you just heard is somthing happening off in the distance or if there is a necromorph about to jump out at you. Also the periods of silence have a very foreboding feeling.
GAMEPLAY..........The difference between Dead Space and other similar games is the emphasis on dismemberment, meaning you can save ammo by blasting off various appendages.
For those of you who are interested in the PS3 trophies, there are a number of them that can affect the way you play the game, such as trying to complete it only using 1 weapon.
STORY............The storyline is very predictable and very cliched. You arrive not knowing what has happened, only to be attacked by all manner of creatures. Mad scientists, alien artifacts etc.... all the story elements have been done before in various games (Resident Evil, Doom 3, Bioshock...). But don't let this put you off as its well worth it.
OVERALL......If you have a sensitive disposition you are probably best staying away from Dead Space. Everyone else has to play this game.