It's one of those movies where the less you see or read, the better. The trailers I watched actually ruined a couple of little things. I made a point of not reading any reviews before seeing it as well. So, apparently, did one guy in the theatre because about 15 minutes in I heard him say, "What is this movie about?"
It's more than just a scary movie. And, it's more than just another movie subverting scary movies. It was co-written by Whedon and Drew Goddard, who wrote some really good Buffy and Angel episodes. Crap. Everything I want to write is spoilery. It's funny. It's scary. It's got Thor, Josh Lyman, and some Whedonverse people I didn't know were in it! It's awesome and then it goes somewhere even more awesome, and just when you think it can't get any more awesome: it does. I don't want to risk ruining it. Just, go see it.