I see what you are saying, and I do agree with you that the heart of the stories had little to do with the supernatural elements, which could have been interchangeable, really. Calling Sixth Sense, a movie about ghosts, and Signs, a movie about aliens would be selling them short, but in the end those elements were in those movies, and while there were only three or four of such scenes in the Sixth Sense, Signs dealt with it from a small town point of view regularly.
At the risk of pulling a D and putting a game into the middle of a normal conversation, I just want to mention the game F.E.A.R. I always thought it was inaccurate to describe it is a horror game, or even a supernatural shooter, like many people did. While it had a lot of supernatural bits in the story and on top, the actual game was about combating super soldiers, who behaved very humanly.
Anyway I see what you are saying, and our only disagreement is whether the fact that the supernatural elements in M Night's movies were interchangeable, affects how we should define them. Let's leave it at that, because I don't think we could convince the other, and it isn't a huge point of contention anyway.
Just to further chat on his formula, his best movies have always had plenty of heart when it came to the crux of the movie, as you put it. While the side plot devices have been good enough to thrill and intrigue a bit.
The Village I felt was sorta where it started to go downhill a bit and I don't think it was just us getting tired of his work.
D, I saw The Happening. After hearing the terrible reviews, I expected the worst, and strangely, started thinking that my low expectations would let me enjoy the film more. That wasn't to be, and I thought the movie was pretty bland overall.