All angles of Von Trier's genius bursting in this movie have been more or less thoroughly explained in the previous posts. I'd just like to add one thing that wasn't much talked about, yet I thing it was the crown jewel of this movie.

Yes, the movie is a brilliant study of characters and (dark) human nature, but more importantly it gives viewer a priceless lifetime lesson. It displays a battle between moral purity, youthful idealism and unaware-of intellectual arrogance on one hand and pure-and-simple concept of responsibility on the other. It is in fact this battle that squeezes and twists our stomach throughout the whole movie; it is this opposition which, in all its clarity, finally unveils itself in the final dialogue. Intellectual exchange of argument defines the "winner". And leaves the viewer contemplating, speechless in awe.

A masterpiece that raised the bar of cinematographic creativity on a brand new level.