The first film was about a young couple, the girl thinks there are ghostly going-on in the house, the boy puts up CCTV, and it picks up the banal domestic daytime activities and then peculiar night time noises and horrid events. In this prequel the girl's married older sister, in the months before the previous film, has a similar experience. The main difference is there are two nice kids and a dog in this house.
I was quite pleased that the cinema was noisy and the plot a bit baggy, because when I got into my seat I remembered how scared I had been by the first film. The impact was quite a lot less this time.
It's interesting how the structure and editing of a film builds or undermines the emotional tone. The structure of this was confusing and dissipated a lot of the tension IMHO.
The scary scene shown in the trailer doesn't appear in the film. I don't just mean the particular shot isn't used, but the sequence combines two dramatic elements that take place three weeks apart in the film which was eventually shown. And during those three weeks the events of Paranormal Activity I are supposed to have taken place. So I am thinking there must have been some major last minute fiddling about between releasing the trailer and releasing the film.
I'm not sure if the original was pretty good, and the studio has mucked it up at the last minute, or whether the original was so very bad that they had to salvage it by hacking it up. I'd be interested to compare what was shown with what was planned.