There's nothing particularly original about The Conjuring. If you've seen classic horror like The Exorcist (1973) or Poltergeist (1982), or even The Sixth Sense (1999), there's little in James Wan's bloclbuster that will surprise you. However, it does recycling the tropes of horror masterworks better than most of its peers.
My favorite kind of horror is that which interrupts prosaic, everyday life, and Wan makes use of a terrific cast to establish a good enough baseline of ordinary reality for old horror tricks to feel meaningful, if not exactly fresh. Unlike the glut of contemporary horror movies that insecurely signal their scariness with overbearingly ominous lighting and music at all times, The Conjuring uses the clean openness of natural lighting and no foreshadowing score whatsoever, saving its scares for when they will matter the most. This seems to me to be uncharacteristically restrained for Wan, and maybe he felt that the period setting wouldn't be conducive to his usual overdose of style; if that's the case, he should make more period horror. It's the best work he's done.