I'm probably going to write a few differant posts on this subject. I will say that this is one of my favorite movies of the last couple of years. It is however hard to really answer some of the comments in this thread, or concerns about the movie and what it might be (going by the trailers) without basically ruining it. As a result the rest of this post can be considerd to be a spoiler, partially directed at people who have seen the movie, and partially directed at people that would prefer to know what it's about before spending money on it (which they should, to support it and further things in a similar vein), either due to not liking Joss Whedon, concerns over the genere, or heck just preferring to know the end of a story before they see it so as not to waste their time. While the surprise being ruined DOES remove some of the fun, it's actually not that deep and I was able to figure out most of it by inferance within the first 20 minutes anyway, even if it was novel.
SPOILER FROM THIS POINT ON:
Yes this is Spoiler-rific (but for a reason).
A pretty good synopsis of the movie is to say it blends traditional "campy" horror, lovecraftian horror, science fiction, and comedy into one package. The basic overall plot is that the "ancient ones"... think of Lovecraft's "Cthulhu Mythos" are asleep below the earth, in order to keep them from returning a ritual must be performed, where specific types of people are terrorized and killed as a sacrifice. The world goverments realize this so for the preservation of humanity you have secret organizations luring people into prepared areas periodically and killing them to prevent this great evil from escaping.
To be entirely fair this is pretty well spelled out by artwork shown during the intro credits, and some of the comments made by the guys in the "secret goverment organization" who are revealed early on in the actual movie, so most of this isn't going to surprise anyway.
If that set up sounds wonky, it kind of is, but it is skillfully directed, and winds up poking fun at a lot of genere tropes. Part of the set up has to do with the sacrifices having to make choices, so the guys running the show have taken a bunch of cursed objects that summon monsters they have imprisoned and placed them under the cabin, and then kind of lead the people inside the cabin to them, where they choose what kind of creature is going to kill them (lesser supernatural creatures exist, and are apparently hunted down and captured). A point interspaced with scenes of lab nerds creating betting pools over what kind of crtitter the goobers are going to set loose on themselves this time, with some rather unsubtle referances to various popular horror movie franchises.
You also have situations where the guys controlling the area where the cabin they lured people to is (which is sealed in by force fields and such) use mind control drugs and other tactics to control the behavior of the people in the cabin. For example there is one bit when the monsters are unleashed where the victims-to-be very sensibly say "hah, let's stay together and barricade the cabin so they can't get in" which of course freaks out the controllers who use hypnotic gas to cause the leader to suddenly switch to "hey, let's split up and cover more ground!" even if it makes no sense (less than usual) in context of the situation... which is kind of where the whole "comedy" thing comes into this.
The movie does a good job of switching between what is actually some very serious horror and science fiction concepts, with very high stakes, and outright comedic insanity, without making the whole movie utterly stupid, which is quite a feat. Also the monsters are pretty awesome, early on you might be disappointed to see some of the options in the lab betting pool that were missed out on... but let's just say by the end you'll have seen tons of horrible monsters, including some rather stylish ones based on the more "WTF" options like "wait, death by Sugar Plum fairy?".
The movie also lays down one of the most amazing (if playfully intended) smackdowns on the entire J-horror genere I've ever seen, that actually made me laugh.
The above spoils the "plot" of the movie, without going into a blow by blow, but might be useful to some people, and I DID include a spoiler warning. For the people who have seen the movie here is a more detailed commentary on a specific ASPECT of the movie:
I'll be honest in saying that I was also less than impressed by the ending, namely because once I realized what it was borrowing from Lovecraftian horror it was kind of predictable, espcially given the generally left wing morality being espoused in the media today, though this isn't really a political rant per se.
See, the whole set up makes it pretty clear that the people in the cabin have to die for humanity as a whole, creating quite a conundrum as the survivors realize the reality of the situation. As a result I kind of felt that the "final girl" stereotype should have shot the stoner, or the stoner, who was basically supposed to be a good guy, should have shot himself. I could have dealt with the ending if they had made some point about breaking the cycle and perhaps being able to defeat this evil in the modern age, and themselves survived, but the whole "whelps, the right thing to do is to die ourselves but do it in such a way that humanity has to face the wrath of the Old Ones anyway" was just plain out F@cked up and oddly enough seemed out of character for the guy who was in the position where he needed to die to begin with.
On the other hand I must say that the ending does leave it open for a sequel of sorts, albiet not as "Cabin In The Woods 2". Oddly with the way the movie ended and thinking of the high camp involved, I was thinking tha maybe Joss Whedon's new project should be to make his take on a giant monster movie (sort of like what JJ Abrams tried with Clover Field). Godzilla and Jet Jaguar (or more appropriatly the equivilent there of) throwing it down with Nylarthotep and Yog Sothoth (or the equivilent there of) in the middle of LA could be kind of amusing especially if done in a similarly obtuse way. I'm probably the only one who thought that would be a fitting "sequel" however (given that both movies would ultimatly be stand alone products with differant themes).
All told, very good movie, if not very deep. The ending is one where you can see how you can have a mediocre ending to a hard to end storyline that manages to not piss everyone off by being that way like ME3 (yes I KNOW some people can defend it, but from people who saw the ending, it's very debatable, so obviously a lot disagree. However notice nobody is pissed off enough to lodge official complaints, and that's a differance, at least Joss made an effort here and the ending did kind of fit even if it probably could have been better), and afterwards when you take your brain out of neutral your going to ask yourself questions like how the goverment coalition behind this that captured/controlled all these monsters and the artifacts wasn't better prepared for a breakout (ie having people standing by with monster busting geaar as opposed to throwing black clad stormtroopers at bulletproof horrors... I mean they had to cage those bloody things to begin with, and get them back in after each sacrifice. Mass breakout aside, nobody did anything remotely competant and you'd espect some plan for a containment failure), or even why with massively more advanced technology nobody was prepared to say drop a couple of gigaton yield warheads down the hole as a last ditch effort if the sacrifice failed... I mean it might not work, but it would have been a nice touch. Basically filling the whole trope of the more massive an orgnization is, the stupider they obviously become.
I might actually be convincing some people to NOT see this movie if they read past the spoilers and were undecided, but hopefully I will have conviced some to see it. In the end though this is very much a really good high camp geekfest, that balances the humor/absurdity and serious elements well, and the flaws with the set up are not going to be so glaring that they are going to beat you over the head when you watch it. For example seeing dozens of unique monsters tearing through security troopers is great entertainment even if it doesn't make sense given the set up if you bother to think about it.
If you respond to me about the ending (please keep it polite, even if you disagree), or other elements, please remember your spoiler tag.