Things that would have made Amnesia scarier

[MysticWind]

IMO, it was not so much the monsters that made it scary. It was the surrounding atmosphere and not knowing what to expect next which is the scary part. Like exploring alot of rooms was very un-nerving, and there ended up being nothing in them. I have not finished the game as yet, but I think the addition of a forest you have to go through would be great (As you saw when walking across the outer window ledges). I think that dark forest would be so un-nerving to progress through, especially if monster spawns were added in the right place. (Or a hunting pack of grunts with dogs)
Also, with the monster spawns.. when you hide you wait until the search music dies away because you know the monsters gone. I think it would be quite terrifying to do that but leave a monster there later in the game, so the player thinks the monsters gone and then wahhhhhhhh
A grunt chained to the ceiling. No one ever expects things from above.
More thorough searches.
As someone has already said, mirrors, but when you look into it you see a monster behind you, leap around and there's nothing there.
I've seen quite a few people bringing up the idea of weapons, give them a gun with a couple of shots, so when they have a monster encounter, they turn out to be blanks