These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
... are worker bees without a queen: they are dim-witted, listless, and have only the two modus operandi "obedient" and "violent". Equivalently, they are bio-engineered soldiers.
... are not inherently stupid and violent drones. It's a reflection of their refugee status: poor, lost, confused, exploited, oppressed, without jobs or hopes. They are only violent and listless as some ghettoized human refugees are.
... were already unwanted brutalized refugees being dumped on Earth to get them away from an "alien Johannesburg."
Anvilicious: The promotional website and trailers drop anvils on racism, illegal immigration, and pretty much anything that can be tangentially related to Apartheid. The movie is a little more subtle. Of course, Some Anvils Need to Be Dropped.
Piet Smit, Wikus's father in law, is a Corrupt Corporate Executive who helps keep the prawns oppressed in the alien ghetto. Initially a cold man, when Wikus is infected and finds himself turning into a prawn himself, Smit has Wikus taken in and plans to have him vivisected in order to study him further, calmly watching his own son in law beg for his life without even a trace of emotion and later lies to his own daughter about Wikus being a criminal.
Koobus Venter is a sadistic member of a special ops team who gets his jollies off murder, and relishes being able to kill without repercussion. Koobus guns down a prawn after it's been subdued and later leads a violent hunt for Wikus, with full willingness to kill whoever gets in the way. Koobus psychologically tortures the Prawn Christopher Johnson while beating him and threatens to kill him regardless of getting Wikus, gleefully saying he can't believe he's getting paid to do hat he loves: he just loves watching the Prawns die.
Obesandjo, The Evil Cripple wheelchair bound leader of the Nigerian gangsters, sets up his racket in District 9 and distributes cat food to the aliens, which is like a drug that keeps them hooked. He also offers "interspecies prostitution," offering them human women. The worst, however, is when he has innocent aliens butchered, so he can eat their flesh and gain their power. When he captures Wikus, he fully plans to cut off his Prawn parts and eat them himself.
Fandom Rivalry: Hardcore D9 fans often face-off against hardcore Avatar fans because of the similar story structure and the fact that both films were sci-fi flicks nominated for Best Picture Oscar.
Hell Is That Noise: The high-pitched ringing-ears sound whenever the earlier effects of the fluid on Wikus become apparent (e.g. when he throws up and when he has a nosebleed).
Hilarious in Hindsight: A pretty hilarious one happens in the Japanese dub of the film: Koobus Venter is dubbed in Japanese by Atsuki Tani, who is the regular dub voice for Master Chief in Japan. Taking into account the film was originally planned as a Halo film and the fact Venter's personality is the complete opposite from Master Chief...
Christopher and his son. Especially his son. Between his own youthful antics, Christopher's fatherly gestures, general tininess, and his various Crowning Moments, the little guy is just asking for a massive hug. Extra-specially when he compares his arm to Wikus's and says endearingly, "We're the same!" Awww... kinda.
The Mini-Mecha has tiny little antennae, like the prawns. Awwww.
Visual Effects of Awesome: Obvious just from the trailer. The aliens? The gigantic spaceship hovering over the city? All CGI. All made on a budget of just $30 million. While Weta Workshop designed the aliens (who, appropriately enough, resemble big crickets), at the time of the movie's post-production, Weta Digital was too busy working on visual effects for Avatar, and made only the mothership and the dropship. Most of the VFX were made by companies based in Vancouver (where Neill Blomkamp lives), with Image Engine taking the biggest part. Though already having worked on other Hollywood movies, District 9 was Image Engine's first big project.
Wangst: Wikus spends a lot of time flipping out and all the whilesaying bad words frequently. He'll come across as indulging in this to some viewers, especially those who think his killing of the baby aliens and his obvious corruption deserved some karmic retribution.
The Woobie: Christopher and his son both come to mind, being pretty much the only unambiguously good beings in the entire movie, and who suffer great hardships.
Jerkass Woobie: Wikus. He's more than a bit of an asshole, sure, but he's been through a lot as well.