Dead Space does a lot of things right. It's not the scariest horror game out there, but it's certainly scarier than its cousin Resident Evil 4.
The main thing is that the atmosphere is amazing. Very, VERY nice science fiction vibe, the ship feels fully realized, moreso than just levels designed by someone, it feels like it was once an operational ship, with a medical bay, hydroponics, a tram system, etc. You really get the sense that you are miles and miles away from anyone else, out in deep space. The lighting is pure creepy bliss, and the sound design is absolutely some of the best in gaming. Not to mention the unique OST filled with creepy strings and blaring horns. The whole game was quite obviously heavily influenced by Event Horizon and other movies like Alien, but it doesn't feel like a ripoff, it feels like a tribute that was perfectly executed.
So that's the main deal, but it also has several other things going for it. For one, I really like how Dead Space does not underestimate the creepy power of loud ambient noise. Just as silence is unsettling, the loud blaring of machinery is often used here to drown out all other noises, so the player can't hear anything creeping up on him. I like that.
And while it definitely had its share of jump scares, people often forget that it could be subtle at times too. Like the time you come across a solitary survivor banging his head against the wall until his slumps over dead. Or moments where you simply see shadows, or hear noises that never amount to anything. And I always remember how when you finally return to the medical bay, all the body-bags that were previously lying around are gone. Which reminds me, Dead Space is one of those games that does backtracking well. Going back to the medical bay felt like playing through an entirely new area.
There's also the hunter, the enemy that is literally unkillable, and you are forced to run from it several times until you get locked in a room with it and several other necromorphs and have to survive until you can get the door unlocked.
It's not perfect though. It does have a lot of jump scares, and after the first time a necromorph pretends to be dead only to jump up at you, it gets extremely old and you see it a mile away. Also the turret sections where you have to shoot meteoroids and later monsters. Those were pretty goddamn terrible and out of place.
Another criticism I have is that there's little to no build up. Five minutes into the game you have a gun and are blasting away at monsters left right and center. C'mon guys, give me some credit, I won't get bored if you just spend a little time atmosphere building.
While Dead Space 2 was definitely a little more action-y, I was surprised to find that the large majority of the game is still spend slowly making your way down dark empty corridors alone. The main problem I had with DS2 was the monster gauntlets. It becomes increasingly evident as you progress that the developer's idea of introducing new gameplay means "spawn 6 more monsters than last time" until you are spending hours just mowing down packs of enemies at a time.
Dead Space 3 looks to be going the way of Resident Evil 6 unfortunately. Monsters that use guns, big action set-pieces and the hilarious bare-faced lie that is "better with Kinect".