Sinister: This is another in the category of "Not particularly original*, but undeniably effective". I think one of the things that really makes it work is that much of the movie consists of Ethan Hawke sitting in a dark room watching videos of horrible murders being committed, apparently by someone from a black metal band, while the situation gets creepier and creepier around him, and that's also essentially what someone watching the movie is generally doing, so it adds an extra bit of immersion.
*It's basically a crossover between the "arrogant dude who always says everything's fine" (e.g. Amityville Horror) and "creepy children" (e.g. The Ring) subgenres. Pet Sematary occupies a similar space, genre-wise.