It's hard to say a lot about this film without spoilers, and apparently some reviewers have been very spoilery - James Moran mentioned one review that spoiled the closing frame of the film in the opening sentence of the review; that's just mean, people. In light of that, I've avoided reading anything about the film, so went knowing only that it was Joss' take on the sort of film that Evil Dead and its kin spawned: teenagers go for a break to said cabin in woods, and very bad things happen.
In terms of the story, I'm not going to say any more than that. Nope.
There are other things that I can mention, without giving things away. For at start, it's very funny. There's Whedon's trademark under-cutting of portent. You know the kind of thing I mean: like in Angel, where a demon would be going through the process of conducting a ritual, full of ancient language and moody overtones, then would get a cell-phone call part-way through. From its mom. There's Whedon's trademark recycling of other actors ("ooh, ooh, it's whatsisface! Y'know! From thingy!"). And it has Something To Say about tropes in horror movies. These kind of horror movies, especially.
The other important thing to say about this Whedon film is that it's not just a Whedon film. Drew Goddard directed it, and co-wrote it. Credit is due.