Violence Despite being basically an interactive cartoon, South Park: The Stick of Truth is extremely violent, both graphically and emotionally. The kids not only beat up on each other, but adults also attack them, and they sometimes also fight homeless people, domestic animals, and others unprovoked. There is a fair amount of blood and gore, including a scene where one kid (Kenny, whose gimmick is that he dies constantly only to be mysteriously reborn) is torn apart and carried away by rats.
Scary Imagery The game isn't scary so much as profoundly unsettling—it is not meant to be a horror game, but it might give players nightmares all the same, especially younger kids.
Sex & Nudity Nudity and sexual content abound in South Park: The Stick of Truth. Anal probes, bestiality, masturbation, sex toys, and circumcision are all graphically depicted (in South Park's signature cartoon style). Players can collect "pubes" (pubic hair) as a sort of prize. Basically anything goes here.
Strong Language If you can think it, it's probably in the game. This includes homophobic, gendered, racial etc. slurs.
Substance Use While I'd hesitate to say the game encourages illicit substance use, it certainly doesn't shy away from it. At one point the kids get into a meth lab, where used syringes and other paraphernalia can be found. Many characters appear drunk, and the kids can use alcohol bottles as weapons.
Consumerism Since the game is based off of the TV show, there are quite a lot of product tie-ins.
Discrimination Because of the nature of satire, it's up to the individual to decide if South Park: The Stick of Truth is inclusive or discriminatory. Negative stereotypes of different groups are par for the course, but knowingly so. Regardless of preference or outlook, it's essential to discuss these (very) negative stereotypes with kids who might not be familiar with them or recognize them for what they are, and discuss why they might be a) harmful and b) funny.