A movie claiming to "turn the genre inside out" is usually studio-speak for a movie that someone greenlit because it "pushes the limits" by having a brunette female lead, or has friendly aliens. In the case of the long-delayed The Cabin in the Woods, "turning the genre inside out" or even "high concept horror" doesn't quite cover it. As long as you're not into the whole brevity thing, we should really just refer to it as "one of the best horror movies of the last decade."
This is the biggest thing to happen to the genre since Kevin Williamson and Wes Craven made us Scream in 1996. It's no surprise that something this important is coming from the guys behind Buffy the Vampire Slayer, Firefly, and Lost (among other projects), which all have the commonality of throwing audiences a life preserver in an otherwise hopeless sea of junk. So here comes the important part of the review: if you haven't seen the flick, don't do anything that will put you in the know--just get thee to a theater. Forget what Schoolhouse Rock taught you--knowledge is not power. It will ruin the magic of discovering this movie, and the ads have already wiped away some of that possibility just to get you in a seat in the first place. So do what Joss Whedon tells you and go see it, already.
What makes this movie glee-inducing is that we've been beaten into submission by modern (boring) cinema so we're operating with rock-bottom expectations (why else would American Reunion be number two at the box office?). This movie takes full advantage of that. Even if you think you know how it's trying to fool you, you can't completely predict it. The first scenes with Bradley Whitford, Richard Jenkins, and lots of lab coats don't jive with the "kids go to the woods and get killed" genre. But these filmmakers have too good a hand to give you any tells, so while you're wondering what you got yourself into, you see hot teens (Chris Hemsworth, Kristen Connolly, Jesse Williams, etc) pack up their marijuana, daisy dukes, and the Jamie Kennedy-esque comic relief (Fran Kranz) into a motorhome and head for a weekend at the lake. This is the familiar territory.
Hitchcock said the audience has to know there's a bomb under the table in order to make them nervous about it going off--you get more mileage out of tension than you do a surprise. Whedon/Goddard, who I now officially want to kiss on the mouth, take that idea to the next level for modern, savvy audiences. Their switching between a tired story and one that you are slowly getting information about actually gives the audience credit for being intelligent. There's so much joy in every little detail. So by the time you've sort of figured it out, laughed a lot, and been on the business end of some good scares, the final nail in the coffin brings the house down. This movie gives me faith that Hollywood is actually willing to do something outside the box in the name of entertainment--like casting redheads, for instance.