Quan Chi's leg ripper
Top stuff from MK4 on the N64. Bald assassin Quan Chi gets a good grip on his victim's ankle and braces himself with his foot as he wrenches off the entire leg - which he then uses to batter his downed opponent to death.
Smoke goes nuclear
MK3 marked the point when the series lost any remaining pretensions of reality. Case in point: Smoke's fatality, in which he spews bomb after bomb out of his chest cavity, resulting in an explosion that takes out the entire world. How do you follow that?
Mortal Kombat has long boosted its tally of playable characters by switching the colors on certain fighters. See Sub Zero, Scorpion, Reptile, Smoke, Jade, Mileena, Kitana and many others. It's one of the defining features of the game, even if it started as a cost-cutting measure.
MK2 also added friendship moves for those who wanted to further embarrass their opponent. Friendships were generally more about humiliation than anything genuinely platonic, and grew increasingly bizarre in later MK games.