The Conjuring got very good reviews and did really amazing business--so much so that there's going to be both a sequel and a spin-off.
And there's a lot to like here: fine, grounded acting; fun, practical effects rather than CGI; some spooooky bits.
But there's also a lot not to like. Here's a spoiler: when the two demon-hunters start to look into the past of this haunted house, they find a murder. OK, that's pretty standard in horror films: people find some horrible history. Sometimes, in some films, the investigator discovers the horrible history and a red herring horrible history. ("Oh no, there was a murder, but this haunting is from this other suicide!")
Similarly, in this film--and many other horror films--the investigators look for some evidence and find some.
In The Conjuring, once the investigators start to find out the horrible past, it piles up almost comically. Similarly, the evidence is so over-the-top that it's hard to take these people seriously. A woman hung herself? Someone drowned a kid? There's an evil doll? Birds keep crashing into one wall of the house? The house is freezing and smells like rotten meat?
At some point, this seemed more like an SNL parody of horror than a horror film to me.