Heavy handed music
I have several issues with this film (not least of which is Radcliff's poor acting) but putting all that aside, the thing that really started to bother me as the film progressed was the really heavy handed use of the music to sign-post things in the film that I'm quite capable of getting myself.
To be specific, when he's wandering around the house in the dark, or working at the desk, and things are moving around behind him - shadows, or fleeting glimpses that were maybe not that cleverly done, but worked OK - the music crashes in with a flourish as if to say "Look! Look! Behind him! Did you see that?"
Yes. Yes I did actually, and blasting the soundtrack with whatever your instrument of choice was doesn't make me notice it more, or make it more shocking, it's just coming across a bit needy - "I really liked the way I did that bit in the room behind you, and I really really want you to notice it and like it too. Did you see it?"
OK, it's just music, and I can usually filter it out if it's too much, but on this one, it got in the way so much that as an illustration of the problem, at one point my Girlfriend turned to me and said "Is that the music or can he actually hear something there?" (He could hear the rocking chair, but it was being drowned out by the soundtrack)
There were definitely chills in there - the hairs on my arms stood up numerous times because there was some genuine creepy moments. But I think the clumsy use of the score highlights that this was basically a weak film, lacking in confidence in what was good about it, and trying to cover for the flaws with the heavy handed use of music to create tension that either wasn't there, or was and didn't need the music to make it so.