Even though the movie as a whole isn't really awesomely smashing in any way, the first thing that really struck me is how well done and thought out it all is.
It features realistic portrayals of human behavior all the way through. From phobias, realistic explanations of how they came to the conclusions they did, to a person using a multimeter and electrical tape to actually fix something. Like a real person might. And they all have their agendas. Also gunshots don't sound like tanks, and they don't make 10" holes in people. There are no huge aliens, there's no gratuitous nudity and it doesn't take one chop with a cleaver to cut someones arm off. Another thing that surprised me is that bodies don't vanish, the same with tracks and bruises etc.
In most movies (*ahem* multi-million dollar productions) stuff like this, commonly called realism, is just glazed over. Your focus is shifted to the CGI and the huge explosions the MTV generation seems to love, like some magician distracting your attention so he can get away with tricking you (out of a good plot).
Anyone who likes realism and thrillers/horror will probably have a jolly fine time watching this one. There's a bit of bad acting on the part of a few characters (though all the main ones are excellent), and as I stated the movie isn't miraculously good in any way. It's still better than most, and it has some really redeeming qualities to it.
