Sareena, who is most likely the best example of this next to Quan Chi. Despite debuting in an unpopular spin-off, there was quite a bit of fan interest to see her return for future games. After several attempts to make it into the series proper, she's finally made playable in Armageddon, where she proves to be a valuableally of Younger Sub-Zero and is made into a top-tier character.
Goddamned Boss: Shinnok's first form. You can only hit him right when he attacks or taunts you, otherwise your attacks will be blocked by his Deflector Shields, which also hurt you if you touch him. And if you take too long reaching him after freezing him, you have to freeze him again. If you freeze him while his shield is up, it will stay up, leading you to take damage when you try to jump over it.
Harsher in Hindsight: After Bi Han returns with Shinnok's Amulet, he asks Raiden about what Quan Chi meant by him being tainted with evil what that meant for his future. The thunder god told him that "But only you can control your destiny. Not even the gods can alter your chosen path in life." These words would likely haunt Raiden in light of what happened in Mortal Kombat 9 in which he sent a message to himself into the past to alter the timeline in order to thwart Shao Kahn's victory in the Battle of Armageddon and ended up unwittingly bringing about several heroes' death in the process.
Shinnok's epically smug trollface seen after he kills Sareena is also catching on.
Narm: The FMVs... are kind of laughable, one example being the cutscenes that play whenever Sub-Zero falls down a bottomless pit. They are quite overdramatic and look very cheap, even using the exact same animation.
Scrappy Mechanic: The transition from a fighting game to an action game brought multiple issues:
Sub-Zero's walking animation is terribly slow. He walks just like in the fighting game. The player can try running but they will become defenseless.
Platforming is incredibly complicated in the second stage. There are platforms that spin 360 degrees and getting to them is close to guessing.
Changing direction from left to right requires to press one of R/L buttons rather than the directional buttons. As a result the player will be defenseless while doing it.
Players who try Easy will feel slapped in the face when the game ends with a cliffhanger and they have to replay the entire game in a harder difficulty to actually finish it.
That One Boss: Fujin, the Wind God, is a pain to everybody. To make it worse, when he is weakened, he tries a Kamikaze technique and figuring out how to avoid it takes some time.
Shinnok can also be this if you want to actually defeat him rather than just stealing his amulet. At first he fires projectiles which don't do a lot of damage on their own, but the number he launches racks up damage very quickly. He is also hard to even hit since when he's no attacking his Deflector Shields will block all of your attack and damage you if you touch it. Taking his amulet causes him to break out his One-Winged Angel where he becomes an Advancing Boss of Doom that kills you in three hits, but his long reach means if he hits you once you get hit as soon as you get back up, meaning that getting near him in instant death. The game expects you run away at this point so the portals that would normally take you to the opposite ends of the stage will take to the ending. This means the only reliable way to beat him is spam the Polar Blast and hope you have enough restorative items to use it enough to beat him before he reaches you.