I didn't find Dead Space very scary, either.
 
A lot of people have been complaining "Oh no! They're turning Dead Space 2 more into an action game!" when Dead Space 1 already was an action game! And a damn good one, too. 
 
It's all for the best, because I'm honestly too chicken to play ACTUAL scary games (I refuse to play a Fatal Frame game EVER).

Just finished the game about 3 hours ago, I was pretty scared and tense throughout the majority of the beginning and middle, lots of it was jump scares. Eventually you've encountered the different enemies enough times that you know how to deal with them and what to expect, I wasn't frightened at all on the last few chapters. Except for that hunter monster thing, scared the shit out of me, probably because I couldn't just shoot it away.

Playing on Impossible mode had me fear every encounter with the necromorpths.

I'm not  usually scared by the visuals, but well made sound design that builds up the sensation of dread towards the scare usually works for me.  When the scare is too obvious it doesn't work and Dead Space had plenty of those "obvious jump scare# moments for me. 
 
In the begining of the game it  had some "there's something behind me" moments, which made me check my back - usually for nothing, but that one time when one of those little tentaclehead baby bastards was there... that really scared seven shades of Shatner out of me. 
 
I was most freaked out the first time when you go out of the air lock and the only sounds you hear are coming from inside Isaac's helmet. Also the zero gravity sequences were very clever, when they made you stick to the surface of the ship and let the necromorphs weightlessly float at you from almost every possible direction. Though the novelty of those sequences wore off pretty quickly...

I'm probably rather yellow-bellied when it comes to these sorts of games anyway, but there's just something about the sounds in Dead Space that get in my head. From the cliche stuff like whispers and odd thumps and rattles to the echoes and grinding of steel against steel, even the sounds of the computer interface stuff have a way of even just slightly altering my mood and putting me just that bit more on edge. 
 
I'm not about to claim it's the scariest game, but it has a great atmosphere and a somewhat unnerving sound design. As far as this breed of action/horror games go I'd certainly say I found it scarier than RE4 (and definitely RE5). I like the sense of empowerment playing as Isaac with his weaponry brings, though. It might not be the best thing for a game that brands itself a horror title, but it strikes a nice balance for me.

Sounds like you were expecting a different game. The game was clearly marketed and advertised as an action game. If you came into this expecting to be a helpless dude running away from aliens on a spaceship, then you bought the wrong game. The game is awesome, but it's not supposed to be like how you want it to be.

The most disturbing parts is probably when you mess up and die, like the death scene on the final boss, jesus christ that didn't made me feel good

" Sounds like you were expecting a different game. The game was clearly marketed and advertised as an action game. If you came into this expecting to be a helpless dude running away from aliens on a spaceship, then you bought the wrong game. The game is awesome, but it's not supposed to be like how you want it to be. "

I'm with you, I thought it had some uncomfortable moments. But not intensely terrifying. Great game though.