Time for some film commentary from someone who never goes to the movies.
I don't intend to see The Strangers, because I don't like Liv Tyler or torture porn, but I will use its forthcoming release as an opportunity to explain my theory of why The Sixth Sense is really scary. First, the trailer for The Strangers:
To my mind, the scariest part of the trailer begins around the 50-second mark, when Liv Tyler is standing in the living room, and a tall, menacing, masked man slowly materializes in a doorway behind her, and then just stands there, unseen, declining to actually menace her for the time being.
What would be decidedly un-scary is if the guy ran into the room going "BLAAGHGAHGHAGH I'M A CRAZY PERSON AND I'M GOING TO KILL YOU WITH THIS AXE!!!" As it is, the idea of Liv eventually turning and seeing him there is what's truly terrifying, and what really hits the primal fear invoked by the movie's title and premise: Strangers. Or, more particularly, strangers invading your house. It's not what the strangers are going to do to you once they're in your house that's scary, but the very fact of them being in the house in the first place. Your house is supposed to be your ultimate safe place. Only you (and whoever else lives with you) have a key to your house. You're supposed to be able to lock everyone else in the world out, and keep your house as a private, exclusive, safe place for yourself. Once a stranger breaks this ultimate security, it doesn't matter what they do because they can do anything. You are 100% unsafe if an unwanted attacker breaks and enters. Plain and simple.
So, what does this have to do with The Sixth Sense? The Sixth Sense is only nominally a movie about ghosts. Well, to be sure, the story is about ghosts, but what's scary about the movie isn't the ghosts. What's scary is, you guessed it, strangers in the house.
Don't believe me? Just pop in the DVD. The very first scene of The Sixth Sense is quite scary and, lo and behold, doesn't involve a ghost. When Bruce Willis spots the broken glass and realizes there's someone in his bathroom, it's scary. As he slowly looks in the bathroom and finds his cracked out former patient standing there, it's also scary, even though at this point in the narrative we have no reason to believe that this guy could possibly be a ghost (and, indeed, he isn't).
Once the ghosts show up, the scariness continues, but not because there are ghosts in the movie. The ghosts are scary not because they're dead, but because they invade Cole's safe space. One of the scariest parts of the film is when Cole walks into his kitchen and finds a woman who he thinks is his mother standing at the counter doing the dishes (or cooking or something). When Cole asks her something and she whips around revealing herself to be a ghost, what's scary is the violation that Cole feels at that moment. He thought he was talking to his mother, doing motherly things, in their house. The fact that the woman was someone else entirely is terrifying in and of itself, regardless of whether she turns out to have slashed wrists.
But, you say, the woman-ghost was a mean ghost! Mean ghosts are always scary! Well, true enough, but what about the kid who accidentally killed himself with his dad's gun? This ghost is scary, and yet friendly. The only thing he says to Cole is, "Hey, come here, I want to show you where my dad keeps his gun," or something to that effect before walking away and revealing that the back of his head is missing. He wants to be Cole's buddy. Despite the sound cue when the hole in the head is revealed, the jarring thing about this encounter is the unexpected presence in the hallway of a strange teenage boy in 70s regalia.
The story picks up on this theme explicitly with the poisoned little girl ghost, who actually goes so far as to invade the little sanctuary that Cole has built for himself. Cole, realizing that his home is not itself a sanctuary (as it should be), has concocted a further line of defense in his bedroom using sheets and religious icons. In another of the film's scariest moments, the little girl (a young Mischa Barton, by the way) slowly pushes her barf-covered face into the sanctuary, sending Cole fleeing out of the room. The moment is drawn out to let the audience wonder whether Cole's last line of defense is going to hold up, or whether he is, after all, completely unsafe from the ghosts. The sanctuary fails, and that's scary.
Furthermore, M. Night knows he's blown his sanctuary-terror wad at this point, as immediately upon fleeing from the little girl, Cole goes back to his room, finds the ghost to be, in fact, a helpless and pathetic (and still dead) little girl, and begins the process of helping her resolve the problem that will allow her to stop not realizing she's dead. This kick-starts the next stage of the narrative, when the scariness is de-emphasized in favor of resolving the story and setting up the final twist.
In any case, I'm not saying that The Strangers is a re-telling or a rip-off of The Sixth Sense. They're two very different horror movies that reflect the horror zeitgeists of their respective eras. The Sixth Sense is a complex, psychological thriller; The Strangers, by all appearances, is, as I said, cheap torture porn. But both films rely on the same thing for their more effective terror -- Watching people get tortured/killed is disturbing, not scary; likewise with watching dead people walk around with unknowingly mangled bodies. What's scary in each movie is the idea of strangers in your house.