The problem is... then the first hour ended, and the movie became a giant chase reminiscent of Michael Bay: all cops'n'robbers and random killing: see more here: http://jseliger.com/2009/08/18/district-9 .
Let's avoid spoilers for those who haven't seen the movie yet, but I must say that I found the action parts very satisfying. I haven't been literally on the edge of my seat in an action film in a long time.
This is exactly my problem as well. I still enjoyed, but was expecting something different, something bigger. Not just another chase film, where one man and two aliens take on an army.
Still the best sci-fi film for a long time. (Haven't seen moon yet).