The set up, of course, is a horror cliche--shades of The Evil Dead, The Texas Chain Saw Massacre, and any number of similar films. The trailer makes clear that something else is going on in a mysterious control room beneath the titular cabin. I won't give away the plot too much, but for those who haven't seen the movie consider this your **SPOILER ALERT**. Behind the goings on at the cabin is a cult of the "Ancient Ones," "evil gods" who wait beneath the earth to rise up and reclaim their domain should certain conditions not be met.
It takes little imagination to see in this the Old Ones of Lovecraft, who wait beneath the earth for the stars to come right again, though in their August Derleth-derived form as emissaries of evil. Nor is this the first time that Whedon has used this trope; in Buffy: The Vampire Slayer the Old Ones were the progenitors of the first vampires and also waited beneath the earth for the time when they could reign again, etc. In fact, the mixture of ancient ritual and modern technology had already been done in many permutations in Whedon's other work, especially Buffy and Angel. The movie's sketchy discussion of mind control and free will was also the dominant theme of Dollhouse.
Ultimately, Cabin in the Woods fails because its somewhat interesting idea isn't given enough time to develop and the situation is forced to an unnaturally abrupt conclusion; the other side of the coin, however, is that given more time (as in a TV series), there would be little to distinguish Cabin in the Woods from any random episode of Buffy or Angel or Dollhouse.
If I could sum up the movie in one line, it would be this: The Truman Show meets Rod Serling's Night Gallery.