We can extend last post's discussion from sex to extreme violence, sadism, body horror ... but keep the focus on how people react and what they think about the author who transgresses taboos in a game or fiction.
A harder job is to figure out why it is done. Perhaps the creator's actual purpose is not so important as the effect it has on the audience. But second-guessing of the creator's motives is always going to go on, and cues in the work may push the audience one way or another. If we just discuss effects, we avoid the tricky problem of the author's intention - is it wish fulfillment? moralization? sheer desire to disturb? - and also confront the possibility that the work might have unintended effects on any given audience.
Effects of isolated shock include:
* Contrast. At the most basic artistic level, the shocking element provides the goal of an investigation or adventure. It gives a dramatic, attention-catching payoff of surprise. In an interactive medium, it can also give clarity to a muddled situation - this thing is clearly an abomination, it needs to go!
* Demonization. A point is taken about things linked to the abomination - sex is bad, drugs are bad, religion is bad. Sometimes, in slippery-slope logic, the greater evil is a stand-in for some lesser form of deviance like homosexuality, race (or racism), unbelief. Sympathies are with the normal community trying to root it out, and if they're oblivious, this just makes a more effective call to arms for the crusade.
* Hypocrisy-bashing. The take-home message here: apparently normal society shares essential traits with the abomination it's so horrified with, maybe to the point of being more monstrous than the monster. This can be done savagely, by making the monster-haters ugly and brutal, or gently, by emphasizing the humanity of the apparent monstrosity. A related theme is to satirize conformist efforts to keep up normal appearances and ignore the monstrous, as in Jaws and many other films.
When shock becomes pervasive, this can convey:
* Existential stress. A "world gone mad" has an artistic effect of unhinging the audience, creating an atmosphere of constant and pervasive threat to one's values and assumptions. Note the difference with contrast. There, the viewer and protagonists have a solid ground and safe space to retreat to, whereas here, both of them are marooned in an existentially hostile world where rules of sex, rules for bodies, rules for eating are profoundly and disturbingly different.
* Dystopia. Endemic wrongness is often taken as the bottom splashdown of the slippery slope, calling out an evil in the world not by isolating it, but by imagining it taken to the farthest extreme. "If you let men marry men, pretty soon, incest and bestiality will not only be acceptable but fashionable!" "The logical consequence of sexism is the owning of women as property!" A fairly standard character in dystopia (as in Brave New World or 1984) is the one character who for some reason is "old-fashioned" and stands in for the audience's sensibilities.
* Relativism. Just as isolated shock has culturally conformist and nonconformist interpretations, so does pervasive shock. The nonconformist version of dystopia leads to a questioning of the very basis of taboos we take for granted, through one of two means. Either the taboo-breakers are portrayed sympathetically ( for example, Donald Kingsbury's SF novel Courtship Rite depicts a harsh, protein-poor planet where cannibalism is normal and institutionalized), or the reader's society is contrasted against the transgressive one in an unflattering light ( for example, Piers Anthony's short story "In The Barn," where an explorer of alternate universes finds one in which humans, lobotomized from birth, are used for meat and milk, and reflects on what an explorer from another universe might think of our own treatment of animals.)
Finally, there are two reactions which have been presumed in both of these genres of shock.
* Wish-fulfillment. Based on the assumption that each taboo covers a deep dark desire, one presumed intention or effect of portraying shocking things is the vicarious service of such desires, both in the author and the audience. Undoubtedly this is the case for some, but how much we really want to break taboos may be overstated. The possibility of wish-fulfillment, though, does make for some good hypocrisy-bashing and relativism aimed at the audience itself. Nobody who's seen Tarantino's Inglourious Basterds can escape its point that the Nazis cheering the imaginary violence at their film premiere are uncomfortably mirrored by the audience cheering the imaginary violence against the self-same Nazis.
* Desensitization. This is less of a goal for authors, than an unintended side-effect. The Technicolor gore of 1978's Dawn of the Dead now looks laughable and primitive; pornography has ritualized a certain kind of sex so much that the only way to shock people now is to present sexual bodies that are hairy, lumpy and ugly. Desensitization has been a concern of regulators and moralists for a long time, exemplified most tellingly in the strictures of the Comics Code of the mid-20th century, which required not just that evil be punished but that it not be depicted overly graphically. This assumes only thing keeping us away from committing vile sins ourselves is innate revulsion, which can be desensitized by repeated exposure, much as medical students get used to the feel of cadavers.
Well, that's quite a scheme there. I'll let that sit, and pick up again with the ways these categories can be applied to confrontations with the past.