Poor choice of music ruined the scares
It's been mentioned before on here but that accompanying music that sounds like the first few chords of a metal song, whenever the demon strikes, really does diminish a lot of the horror of those scenes. Whose bright idea was it to use it? If you've ever heard The White Stripes' The Hardest Button to Button' it kinda sounds like the intro to that playing on repeat. They actually stop using it halfway into the movie but by then it doesn't matter as the second half isn't as scary anyway with the introduction of the parapsychologists and the capturing the entity with liquid helium plot, which is ridiculous by the way.
Thought they could have built up the tormenting more gradually than begin with a full on rape as well. Aside from that and the above issues it's a decent movie overall.