One of the things that makes the movie feel so creepy is the extensive (but subtle) use of subliminal images. For example:

In the opening scene, a car drives along a winding forest road as a thunderstorm animates the sky and drenches the countryside in a deluge of rain. The camera is situated in the forest itself, some distance from the road. Watch the trees in front of the camera-- as the lightning illuminates the trees, you'll see the shadow of a man holding a sickle above his head. Almost too quick for the eye, but definitely enough to start setting the nerves on edge.

The movie is heavy on atmosphere and rich in detail, something utterly lacking in most modern horror flicks.