Bodies strewn about, sometimes housing an enemy type.

Only problem is that if the player has no knowledge of when a body has an enemy inside of it, it could easily make them feel like they're being punished for something they can't prevent. This can be done right, but I just wanted to give a word of warning to limit this to a somewhat fair experience. (player being given a chance to escape if they accidentally trigger a confrontation.) Sounds like it'd make me shit my pants though, and it definitely would make me feel uncertain/uneasy whenever I'm around dead bodies. I like it.

Having the players decide between fast areas that render you unable to attack(or use some attacks), and slow areas where you can.

Oooh, i like that idea, especially if the player is forced to make that kind of decision in a snap, it could add some well-balanced tension to certain moments.

Like music, I find emotions are best presented in themes. A melody and then a crescendo is cool. Repeated, slightly altering melodies slowly adding up to a crescendo before a huge climax is pretty much the scientific formula for "goosebump" music.

It would be cool if maybe the beginning of the game saw the player with a couple of allies with weapons. The allies fire at the big invincible SCP type thing and do no damage before dying. The first level would be the character escaping this. Then they could get to a safe point without being attacked by monsters. There is nothing scarier in that type of game than being in a safe place and being forced to leave to cross an area with enemies you know you can't kill. This invincible bad guy (along with other monsters) should show up in larger amounts as the game progresses. Each time, it should feel familiar, yet unexpected. They know what to expect -- the fear. The unexpected should be occasionally giving new reasons to fear leaving the safe zone.

I don't know if this is still of use, (Registered specifically to reply several days ago, but there was a problem with my account) but if your monsters are shadows, having them blend in perfectly with innocuous shadows cast by objects in the room would be ideal - allowing convoluted routes around some of them more so, causing the player to weigh up risking injury/death in the dark or going to extremes to stay in the light - much like the character would be doing.

Alternatively or additionally, adding some more monsters in, identical to the regular ones, that behave in exactly the same way but cause no damage or other effects to the player, thus simulating a 'jumping' at shadows feeling.

Oh yeah, maybe provide a few very well lit points in which the player is safe and enemies can't enter, just in the middle of a room, directly under a light or something. Even if there aren't any visible enemies, it still means that players might hesitate, and then that means that they have to force themselves out into the darkness again.

Personally, I dislike the idea of relying on controversial or disturbing topics to disturb the player, at least relying on these things specifically or too much. Admittedly, when it's well done it really IS well done, but there's nothing quite like having elements of gameplay, or the player themselves, causing the unsettlement. (Is that a word?)

Anyway, I had other ideas, I think, but it's four in the morning and time for bed, but I just remembered that I never posted. Hope it helps in some way.

Leziath.

This could happen in reverse of what you would think. For example, you could make it silent instead of noises you hear. Have people die in front of the players, instead of just the player dying. Let the character see the entity from time to time (mirrors, shadows, etc.) but don't let them die right away. The key is to make the players want to quit before the game is over. Maybe as the character is getting closer to the entity, a noise increases. Make it an odd noise like a buzz, or a beep. This would really unsettle me. Also, make the character appear in different places. For example, say the player sees the entity in front of them. Then, when they turn around, make the entity transport to in front of them again. Make more than one later on, so they think they've conquered their fear of one, then they see something different later, making them unsure.

I actually like a lot of what was posted here. Some of the aspects, like silence and loss of direction, I had considered before but in a different way; rather, I saw them as a means to subtly hint at things coming up so that the player could build up tension. Some things however - given the constraints listed above (mainly the 2D perspective) - would make it harder to implement.

One way to help with that would be to partially shift the camera to the direction that the player is facing much like Cave Story does (seen below). At about 0:52, you can see the camera shift forward to face the player's direction, giving them more space. Given that my game would be more puzzle based and required some decent amount of character movement, this similar camera style could be useful and provide what NoAdmiral mentioned (not knowing whats behind you as much). This might also tie in with the enemies that slowly follow you.
VIDEO HERE

I don't know if this is still of use, (Registered specifically to reply several days ago, but there was a problem with my account) but if your monsters are shadows, having them blend in perfectly with innocuous shadows cast by objects in the room would be ideal - allowing convoluted routes around some of them more so, causing the player to weigh up risking injury/death in the dark or going to extremes to stay in the light - much like the character would be doing.

I find it very cool that you'd register just to reply to my thread! And might I say that this point also intrigues me a lot. When I was thinking of how to present the monsters, I thought of it in a traditional platformer sense in that they simply existed and the player knew that. However, this idea of the enemies hiding in shadow could result in interesting presentations. Primarily, it serves as a mean to cause unsettlement, but also would serve as an interesting way to introduce them, rather than breaking gameplay with a cutscene.

Opinions? Thought's on my input?
I'm very glad that this discussion can keep going, and I believe that it can be creative fuel for many.
Love to hear more, and don't be afraid to critique other member's theories. Multiple angles on the same issue are always welcome.

-Cheers!