As a big fan of horror it's pretty easy to get bogged down in a lot of the crud that comes out of the pipeline, and, while it has its flaws, this movie certainly hits all the little bits that get my spookilies all dookilied.
-Does a great job of either making you watch the entire shot or focus on the one spot you know something is going to happen in. From the long, drawn out scenes where there's an empty, dark, hallway that you know something is going to appear out of, or a fairly busy scene where you know something has to be happening that you just can't catch, they definitely get the anticipation going.
-The"Haunted House" vibe makes a fantastic setpiece for a horror movie, and I feel that it gets more freedom to live up to this aspect than it did in the original. The consistent wondering of "is that a prop?" helps to keep you guessing.
-Really the proper use of the found footage format. The momentary blindness as the camera lens adjusts from a light to dark area is very well executed to add to the tension.
-Like the first, the best scares are all simply done practical effects. A figure that wasn't there before, one that was there but now isn't, a moving of a head or arm of a mannequin. The lack of a "sting" or other audio cue helps to accentuate the idea of "has it always been like that?"
-The scares where they went away from their strong suits really took me out of it, particularly with a couple uses of fairly poor CGI near the end. I understand that some things can't be done practically, but there might be times where it's worth it to cull a scene or edit a shot if the CG just won't work.
-Like almost all horror movies they really struggled with the ending. Hitting a crescendo while maintaining an aura of fear is hard to balance out, and it's no wonder that most horror movies struggle to hit that mark. I'd say that most of my issue with the ending is that A) They took the mystery out behind the shed and shot it, giving away all of those juicy secrets that were fun to ponder on and made the movie creepier, and B) The secrets were all fairly predictable. Without spoiling anything, I'll just say that, yes, your theory was probably right. One of the twists to the story was fairly unexpected, but it rings a bit too much of the "Grave Encounters" movies for me to give it a full "good twist." It certainly does make more sense with Hell House though, so I'll give the filmmakers credit for improving on an idea.
TL; DR: A great horror movie to get you in the Halloween spirit. While it suffers from a lot of the drawbacks that almost all horror movies do, it definitely makes up for it in the scare factor. Now I'm off to see the first one again to see how they compare!