One of the things that makes District 9 so intriguing is the uniqueness of its premise. When I think of an alien movie, I think of the typical sci-fi thriller where aliens come to earth and try to conquer humankind. District 9 is much different though, with its documentary style and aspects of comedy and drama mixed with action.
This unique blend comes from the aliens in this movie, rather than the humans. In most alien movies, the aliens are clearly superior to the humans because of advanced weaponry and knowledge, leaving the humans at their mercy. District 9's aliens, however, are at the mercy of humans as they are almost like prisoners of war, stuck in District 9 and unable to leave. One gets the impression that most humans would like to wipe them out are unable to do so because of rights groups and public relations. So in this sense, the aliens are the victims rather than the aggressors, an interesting departure from the normal alien movie.
The aliens in District 9 are not humanoid aliens, but they are at times humanlike in the way they act (and their humorous human names). Many of them long to get back to their spaceship and go home, but are unable to do so. The aliens that the film focuses on are an alien named Christopher and his little son. Christopher often displays humanlike emotions throughout the movie, which is not typically expected from aliens. As we watch his struggle to get off Earth, he shows signs of loving and caring for his son as a human would. We also see that he is kind enough to help Wikus despite the fact that Wikus was trying to evict him earlier. Finally, we see his sorrow when he discovers his fellow aliens are being dismembered in labs so that tests can be done on them. Such a wide range of emotions is unusual for an alien, and at times he seems more human than those trying to evict him.
Other things that make District 9 powerful are the historical people that the aliens represent. One could argue that they represent the Jews in the Holocaust. District 9 is similar to the ghettos which Jews were forced to live in during the Holocaust. District 10 can be seen as a concentration camp (Wikus even says so himself) which the aliens are being forced to move into. Another idea is that the discrimination against aliens is similar to the discrimination that black South Americans faced during apartheid. It is no coincidence that the setting of the film is a place where severe racial persecution has occurred for decades. Whites treated the blacks as humans treat the aliens in the movie, which I thought was very interesting.