Quite simply, the game that changed the gaming industry.
Mortal Kombat was first released in 1992, a time when the differences between Nintendo and Sega were that one had Mario, the other had Sonic, and some multi-platform games were better on one system than they were on the other, though neither had a monopoly on the best versions. A time when violence in video games came in two forms: stomping the enemy into a cartoonish pancake or shooting them with balls of light until they became a cartoonish explosion. This time was gone the minute Mortal Kombat hit the arcades and the consoles. It introduced bloody, horrific violence into video games, including ripping people's heads off and presenting their skull and spine to the player, punching people in the balls, turning into dragons and chomping their heads off, and impaling them in spikes... and all of it done with characters more photo realistic than any that had been seen before, as well as more blood that had ever been seen in any video game, especially in the censor-friendly United States.
But beyond introducing blood, gore, and plenty of sadistic fun to the world of video games, Mortal Kombat began a chain of events that would reshape the gaming industry forever. This chain began with a backlash against the gaming industry headed up by several parental groups and many grandstanding politicians like Senator Joseph Lieberman. They wanted to censor the gaming industry and legally force it to remain a kiddie friendly medium. This first event had several repercussions. The first was that it forced Nintendo and Sega to decide whether or not they would insist that the versions of Mortal Kombat that would be ported to the Super Nintendo and the Genesis (respectively) be censored. Nintendo, which had fostered somewhat of a wholesome image and wanted to keep and nurture that image, censored the game. Sega, which really lacked any kind of a definite image in the mainstream media or even among gamers, made the bold decision to censor the blood on the surface, but leave EVERYTHING available via a simple code. This created the first real division between the two consoles, as well as between all consoles that would come to the market in the future. Nintendo became a family friendly company that insisted on having family friendly games on its consoles. Sega, on the other hand, gained the image of the sexy, violent bad boy of the gamin industry, and it got a huge shot in the arm from that. Wave after wave of violent, bloody action games arrived on the Genesis, and all of them were staying as Genesis exclusives with no plans for being ported to the Super Nintendo, which started pumping out more and more family friendly games in the vein of Yoshi's Story and Super Mario RPG. The release of Mortal Kombat led to consoles having IDENTITIES, tailored to different age groups and parts of the population. This new trend would go on to shape the consoles of the future, with the family friendly Nintendo 64 going up against the schizophrenic PlayStation that had something for everyone (including the teenagers that discovered the "survival horror" genre in Resident Evil), and now the three way battle between the family friendly GameCube, the equally schizophrenic PlayStation 2, and neophyte X-Box that's hoping to foster the same violent, sexy image for itself that the Genesis had in its glory days.
The second major repercussion was the ongoing legal battles and political threats that the gaming industry has had to put up. With the political backlash that resulted from Mortal Kombat's release led to the ESRB, a voluntary industry rating system that rates the content of games in order to better educate parents on what their kids are playing, as well as to avoid legal threats from politicians and the sue-happy victims of teenage violence (regardless of its relation to video games). The legal threats, however, did not stop, and now they have led to legislation. At the time of this writeup, laws are being passed in several cities that force would make it illegal to sell or rent violent video games to minors. And not only are these laws basing the illegality of specific games on the "Mature" ratings assigned by the ratings board that was created by Mortal Kombat (the ESRB), but the law was upheld by a District Judge in Missouri after he reviewed four violent games, among them Mortal Kombat. The ruling by this judge also declared that video games are not free speech (not "not protected speech", but instead fully "NOT FREE SPEECH"), which is a declaration that is already being referenced in anti-gaming bills in the US Congress. And perhaps just as importantly, the laws that are currently being upheld may be the final nail in the coffin of the arcade industry in the United States, because they would force arcade owners to throw out most of their most lucrative games, many of which will probably not have paid back their investment at the time that they are thrown out. So not only has Mortal Kombat contributed to the ESRB, several laws against violent video games, and a possible federal law against violent video games, but this writeup may be appended in the future to attribute the death of the arcade industry to one of its most successful games ---- Mortal Kombat.
The last change in the game industry that was caused by Mortal Kombat was the inclusion of violent video games as a legitimate form of gaming, which necessitated the creation of several new genres of games. Infamously violent PC games like Wolfenstein 3D came long before Mortal Kombat, but none managed to bring attention to violent video games and show that they could sell extremely well regardless of the platform that they were on. Mortal Kombat's success led to Japanese video games generally being less censored in the United States, such as in the case of the Castlevania series. It also arguably led to the creation of video game genres that are primarily focused on blood, gore, and extreme violence, such as the Survival Horror genre that has produced great games like Resident Evil and Silent Hill, as well as the Crime genre that has brought us the best selling game of 2001: Grand Theft Auto 3.
It could be argued that Mortal Kombat led to games growing up and having the freedom to be as dark and serious as other popular mediums, such as television and movies, but I wouldn't go quite that far. But without Mortal Kombat, the gaming industry would definitely be in a completely different form than the one it has today. Mortal Kombat, quite simply, changed the gaming industry.