I would rate this higher if I knew that this concept hadn't been done before. I do not know that, however, and in a world where everything has been done before, I suspect this one has been, too.
The core concept is just remarkably satisfying to see untangle. Some non-concept and conceptual brethren films are easy to spot within both the script and general visual aesthetic, ranging from Triangle to Inception to Insidious.
I guess I can see why people would rate this poorly; the deeper you get, the hazier the plot points become. For me, however, it is an acceptable level of haziness. It's kind of incredible that the plot remains as coherent as it does.
While a ghost story, Haunter turns the typical haunting and turns it on its head. (I don't think the mirror motif that runs throughout this is accidental.) Abigail Breslin plays a dead girl caught in a day-long loop. You could point to Groundhog Day as the boring comparison here, though other movies have made use of the concept (especially in the horror genre) that points closer to Haunter's tone (the aforementioned Triangle, Salvage, probably others).
But what all of those movies lack is showcasing a haunting from the viewpoint of a ghost. But Casper this ain't. Haunter shows a really creepy version of death that does about as good of a job depicting the "life" and thought processes of a ghost as anything I can imagine.
Of course, such grandeur comes with a big caveat. For a horror film, Haunter is going to lack the scares that most people look for in horror. Oh, it tries to stir some sense of horror within the viewer by nailing the atmosphere and dishing up some nice jump-scares. But we all know that neither of those things truly delivers what horror fans of 2014 look for in a horror experience.
I'm willing to give it a pass on that, though, largely because the concept--and execution of it--really struck a chord with me.