The setup was fairly unexpected, and for that, the movie deserves some points. I didn’t expect the movie to weave in between a documentary storytelling style, and a third-person account of the story’s focus, Wikus Van De Merwe (played by Sharlto Copley, a standby actor in Neill Blomkamp movies).
I also did not expect the aliens to show up as early as they did in the movie. District 9 defied convention, and showed it self to be a different animal than the typical alien-thriller/alien-horror style film. Again, it’s deserving of some points in that regard, though by nature it is more of a Sci-Fi than a horror film.
The setup allowed for a lighthearted atmosphere, at least early on. However, if Blomkamp was so much as suggesting that the average viewer couldn’t pick up on the heavy-handed foreshadowing (the aliens are dangerous; gee, who saw that coming?), he was clearly equating the viewing audience with a dull knife. And that is the problem with a lot of movies, regardless of genre; they’re patronizing and don’t expect you to try to piece things together all on your own. District 9 is not the primary example of this, but you have to admit that its message isn’t particularly subtle.
And, speaking of dull knives, Wikus may very well be one, at least when it comes to the utter lack of caution with which he deals with the aliens. It is clear that he relishes in his work, but there seems to be absolutely no reason why he shouldn’t be going about his duties with a little more care (except for plot convenience).