I was describing to my friend what the game was like, as I was playing it.
I was currently sprinting along a hall way being chased by a regenerating necromorph, and I said to him:
"The game is not really all that scary, it's just tense."
And I think that's pretty much true.
You aren't scared, nothing forces you to hit pause and stop playing out of fear, instead you're just a little taut trying to anticipate when the next Necormorph is going to leap out of that all-too-familiar vent.
The first time you go through a maintenance shaft you aren't scared, just tense. Thereafter that tension disappeared because you realise that - if you experiment with controls - can't shoot or aim in there, therefore you know you won't ever have to fight. You can't crawl with speed, so you know you won't have to run.
You spend the entire game trying to anticipate, until you become familiar with the triggers at which point all that tension disappears and it becomes a stock-standard shooter.