Saw some reviews criticizing the dialogue delivery in the movie. Just saying, it's VERY obvious that's an intentional decision by the director. That's what's so special about the movie and the world of these characters. Just as much as the storytelling is unique and different, these characters and the story are very unusual and don't reflect real-life. And they're not trying to.
*spoilers*
Something about the characters is unlike human beings. They're too practical, at times cold-bloodedly so, which makes them very hard to relate to: the boy, near his death, tries to please and bargain with his father to spare his life; the sister is more concerned about an MP3 player than even her brother's life; and the wife, very calmly and coldly, talks about killing one of their kids.
There's something very off about them. And the dialogue accompanies and highlights that quite well. Plus it adds to the eerie, disturbing feel, something movies like these want their audience to feel.