Downer Ending: Though it's played in a lighthearted way, the end is pretty bleak if you think about it: the wave of possessions has reached the capitol city of Chryse and the police are called up to fight them. In all the excitement of going to war, everyone seems to have forgotten that there's no effective way of fighting the ghosts or even slowing them down for long, aside from illegal drugs which aren't likely to be in adequate supply.
Drugs Are Bad: Subverted; the fact that she's taking "clear" (and was given a dose immediately after being infected) allows Mel to fight off the alien spirit possessing her. Played straight with Dos, who gets high off a nitrous-oxide breather and chops his thumb off.
Fingore: Dos amputates his own thumb while trying to make a food can grenade. That'll teach him to do drugs and precision work at the same time. Desolation laughs at him for being a dumbass.
Foregone Conclusion: The beginning of the movie shows the train arriving back at the capital and Melanie is the only passenger on board, foreshadowing that she's the only perceived survivor of the mission.
Guns Akimbo: Apparently in the future, everyone will be ambidextrous and walleyed.
Heroic Sacrifice: The guy who locked himself in a truck after being possessed by a Martian kills himself when it becomes clear Melanie isn't going to stop trying to free him. A rather unusual case for the film, as every other case of Martian possession causes the victim to be incapacitated (usually in a trance-like state) until they're either converted or expel the Martian. His choice of venue also has the added effect of trapping the Martian ghost in an airtight container.
Idiot Ball: Multiple people pick this up throughout the film, especially Kincaid, by splitting up and going off on their own, and killing the bodies of the miners, even after they know that killing the host only releases the ghost to possess someone else. Ballard even orders the survivors to go and detonate a nuclear power station as they begin to leave, resulting in everyone but Ballard and Williams being killed, and the ghosts' host bodies being destroyed...so they can then swarm Chryse in the form of a sandstorm.
In Medias Res: The film is a recount of events told by Melanie Ballard, which also jumps back to previously seen events to show them from another perspective, to emulate her recollection of events.
Invincible Villain: The titular ghosts are just that - intangible ghosts who possess humans to interact with the world. The spirits can't be killed by any known means (they even tried a nuclear detonation, which did nothing), which means that if their host is destroyed they'll just move on to the next body. The movie dances around this issue by setting up the all-out battle to occur after the story's events, but it's impossible to maintain any hope for the surviving characters because victory is ultimately impossible. Then again, Melanie shows us that getting the entire planet high as balls might do the trick. There's also a certain percent of the population that's naturally immune, so attrition might work, too.
It's The Only Way To Be Sure: The team decides that the only way to deal with the plague of possessions is to blow up the nuclear power plant and destroy the town. This doesn't go as planned.
Lady Land: The human society on Mars is explicitly stated to be a matriarchy, and women are primarily seen in powerful positions. Doesn't stop the men from acting like machos, though.
Matriarchy: Specifically of the "Sexy Matriarchy" variant, which describes the matriarchal society of human colonists in the movie pretty well. While women hold the majority of power, it's mostly a leather-cladFetish-Fuel Future with dominant lesbian leader figures, and women sleeping their way to the top by getting it on with their superiors is seen as expected. The men are still machos, though.
Nigh-Invulnerability: The Martian aliens are body-hopping spirits. Their only real weakness is that their hosts can be destroyed, stopping them at least temporarily. Some people are naturally immune, and drug use seems to stop them from successfully possessing a host.
Our Ghosts Are Different: The Martians exist as an indestructible cloud of red gas which can possess most humans. Once possessed, the humans either don't feel pain or are simply badass enough not to care, as they ritually scar themselves before going on a killing spree against the new occupants of the planet.
Skewed Priorities: The team is trapped in a ghost town filled with Ax-Crazy possessed humans and, for the moment, they're reasonably well-fortified in the local police station. So naturally, Jericho decides that now is as good a time as any to see if he can seduce Melanie (if one could charitably call it seduction) by leading her to a closet where they won't be interrupted. Surprisingly, this would have worked had Kincaid not gotten trigger-happy at that exact moment.
Space Police: Most of the main characters have this occupation.
Too Dumb to Live: The team had a possessed man safely locked up in a jail cell. Bashira Kincaid decides to shoot him through the bars because "Fuck him, whatever the fuck he is". She does this despite the fact that it has already been established that killing the possessed only frees the possessing entity, rather than destroying it, thus allowing this particular one to temporarily possess Melanie.
Unreliable Voiceover: Melanie isn't entirely honest in her recollection of events, understandably so since she's a drug addict and at an inquest concerning the fact that she failed to do her job properly. Most of what she says is true but she lied about being asleep when Desolation Williams made his escape. She also omits her drug use consistently.