The films do not break new ground necessarily but what horror film does these days? Most of the scariest ground has been trod and trod again. The trick is to do it well.
How does Sinister do it well?
First of all they keep the plot simple, they do not waste an enormous amount of time explaining what is going on or explaining what needs to be done to stop it. The script is pretty minimal. How do you get by with minimal script?
One way is to hire good actors. Everyone in this film is good. Everyone is a pro. You don't roll your eyes once. You buy it and getting you to "buy it" in a horror film really is the trick.
This isn't to say complicated horror films are impossible, nor is it to say minimal always works. Yet when faced with small budgets you have to go with atmosphere and you can talk atmosphere to death.
This film creates atmosphere. It isn't perfect, there are parts of it that seem intended to be scary or creepy and simply are not. There is even a small part that seems designed for no other reason than to give Fred Thompson a wee bit more screen time. But it all fits together without a seam and creates just enough of a creepy mood to make you jump once or twice.
Sinister also deals with the fleeting nature of fame and success and what people will do to get it back. But, like everything else in the film, this is kept minimalist. It doesn't leap out at you every second but you realize the main character, played by Ethan Hawke, was once a big deal and now isn't. And he is willing to move his family into a "murder house" to get that fame (and fortune) back. You also see that, even though he says it is for the family, it is really just for him.
And what happens to selfish people in horror movies? It varies from film to film but it is never anything good.