I don't find it scary.
The first game was as close as it got to being scary. I will agree with you that Isaac's silence, and only audible noises made were his breathing and screaming made things creepier. I also think they did a better job with the atmosphere in the first game.
Sure, you can say the game is an Event Horizon ripoff, and that the "atmosphere" in the first game was just Uwe Boll flicking the lights on and off. But I think it was more than that. For awhile, seeing the other 'survivors' as you explored the station was genuinely unsettling. Such as the "Man facing the wall" or "Hysterical woman cradling a very dead and decomposed corpse". There was a bit of subtlety with Isaac's crumbling psyche.
That and the necros were not pleasant encounters either. I played with headphones on, and the noises they made, the token-horror-movie-STARTLE-sounds whenever you laid eyes on one. If anything, it made the necromorphs nerve wracking and tense to fight.
That was, of course, before the "scares" became predictable and I was hemorrhaging ammo from my anus.
It was marketed as a survival horror, and carried itself like a survival horror and seemed to me that it had no idea the consequence of giving the player tons of ammo, tons of powerful weapons, and tons of aggressive enemies meant translating the game into "ACTION". Still, I believe it had a pretty good thing going.
Pretty much all the appeal from the first game was lost in Dead Space 2. Perhaps the worst of which to me, was that the subtlety of Isaac's insanity was now gone, with the game occasionally careening into "Crazyworld" with skulls and shit everywhere. To be honest, I laughed my ass off during quite a few of these sequences. Safe to say that once this happens, the scariness is gone entirely. And what a shame too.