Spoilers abound, so for those of you who don't want to know, well...this'd be a good place to stop reading.
This would also be a good place to point out that the ad campaign for the movie was actually really bad about that. Not just in the sense of, "Gee, the absolute best ad for this movie would be a big black screen and a voice saying, 'We'd love to tell you more about 'Cabin in the Woods', but if we told you, we'd have to kill you.' Then a rapid flash cut of screaming people and zombies and a shot of the titular cabin, all going by almost too fast for the eye to follow, and then back to the black screen and the voiceover saying, 'Kind of like that.'"
But even by the standards of this movie, which delivers its biggest, most high-concept twist right at the beginning so that you can't possibly show anything from the film without spoiling it, the trailer offers huge spoilers. Marty, the stoner character, is "killed" halfway through the movie, but the trailer had several sequences of him in the secret base with Dana, and we hadn't seen those bits when he died. So the big reveal that he survived was deflated. Likewise, scenes that hinted at the true explanation behind it all were shown in the trailer, which was kind of a mistake.
But it's hard for me to complain too much about the trailer giving away all the spoilers when the movie is essentially a spoiler-factory. The more I think back over this film, the more effective I think it would have been if it had never shown us anything that the characters didn't already know. The scenes inside the facility are funny, don't get me wrong. They're a wonderful set of inside jokes for those of us who've grown up on a lifetime of horror movies. But to me, it feels like they traded $100 million in drama for a cool ten grand in humor. All of the reveals to the characters--the mysterious whispering voices, the wire in the lamp, the force-field that kills Curt, the hidden elevator in the grave, and finally the revelation of the endless rooms of monsters--every single one would have been more powerful and more gripping if we hadn't already known about it for an hour. (The most egregious one is the hawk flying into the force field. That scene is such a small pay-off, and it absolutely wrecks the big money shot of Curt doing the same thing later on.)
This isn't to say that the scenes in the lab aren't worth keeping. Ten grand in comedy is still a lot of money, and some of the film's best gags (the big board, the betting pool, "Am I on speakerphone?") involve the office workers. Honestly, I'd really like to see a version of this on DVD that has a few extra scenes shot or reshot to provide coverage for the gaps, and that lets you watch it with or without the office scenes, because I think that's what would make this movie the strongest. For it to really work its best, you need to see this fragile, scared group of human beings getting killed off one by one, and then finding out along with them that the whole thing is staged with the dedication and elaboration of a ritual...and then go back and watch it from the point of view of those who staged that ritual, and find out that to them, the whole thing is as banal and repetitive as a typical day in the workplace. You can get that in your imagination, of course, but it lacks the visceral impact I think you'd get if you weren't being shown what's going on behind the scenes.
(And, I have to say, even in the movie as shot, we could do with a bit less behind-the-scenes, or a bit more variation to it. There were too many scenes of Wendy reminding everyone how important it was not to screw up, and Truman standing around looking scowling and disapproving in what had to be the movie's most thankless role. "OK, Brian, your job is to stand there and frown at people, occasionally interjecting a line about how serious this all is!" "And in the climax, I--" "Die horribly without any lines, yeah. Ready?" These made the office scenes seem a lot more like filler than they actually were.)
But as so many people have pointed out, the action climax at the end is truly spectacular, and Sigourney Weaver steals her surprise cameo because she's Sigourney Weaver and she is awesome. That said, I'd be lying if I didn't have a huge gripe here, too. The whole point of the movie, plot-wise at least, is that the reason people in horror movies behave stupidly and unrealistically and follow movie cliches and always do the dumbest thing possible and always fail to get any breaks... (and how did we not get a scene of the motor home not starting on the first try? The ultimate horror movie cliche, the car not starting when you need it to even though it's in perfect working condition, and we didn't even get one "rrr-rr-rrrr"?) ...isn't because of fate or chance or character flaws, it's because someone is actively stage-managing things behind the scenes to make sure the outcome is pre-ordained. Nobody really would be dumb enough to split up like that. Nobody really would go out for a walk in the woods in the middle of the night. Nobody really would read the Latin out loud.
So why, oh for cryingoutloud why do the bad guys actually have a Big Red Button whose only basic function is "Kill everyone in the base"? Real people do not do this. Real technicians generally don't install a button whose only conceivable function is to cause the death of the user and everyone in the same building as them, even in buildings that don't expect to get unfriendly visitors and even if you have to flip a little switch before pressing it. The Big Red Button is nothing but a lazy action movie cliche, without logical explanations, in a movie whose whole function is to suggest that there is a logical explanation for all those cliches. The only way this makes sense is if the sequel is a bunch of Ancient-Ones-cultists standing around their secret monitors, commenting on all the ways they're manipulating the guys who manipulate the other guys. (Which, okay, would actually be pretty awesome. But I don't think it was planned like that.)
Probably this makes me sound a lot grumpier than I am about the movie. I did like it, and it was a fun experience. But I think that a lot more could be done with this idea. I feel like Whedon and Goddard didn't really swing for the fences, that they were so happy with a movie that got their big high-concept horror movie idea out there that they didn't really work at taking it as far as they could go. I can understand that to some extent; it is a great high-concept idea, and they do some pretty audacious stuff with it (again, complaints about the Big Red Button aside, the climax to the film is about as good as the ending to a movie could possibly be. And the very end was ballsy in a way you don't see very often, even in horror movies.) But I hold these two to incredibly high standards, and I think they could have made this movie even better. Still good, but could have been better.
These are my thoughts on 'Cabin In the Woods'.