These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Americans Hate Tingle: The franchise is infamously unpopular in Japan, not only for the extreme violence and dark nature, but also for its wishy-washy East Asian nature, to the point none of the games have seen a release there since Mortal Kombat 3. It also caused many other Western fighting games to be considered as such, until Skull Girls came out.
Other Base Breakers are Quan Chi, Rain, Frost (although these two are Ensemble Darkhorses), Bo' Rai Cho, Sheeva, Kira, Kobra, and Jarek (the latter since Armageddon).
This finally (and surprisingly) happens to Sub-Zero and Scorpion for different reasons. Scorpion because he is Ed Boon's favorite and Sub-Zero (Kuai Liang), mainly for being a good guy unlike his brother (Bi-Han) and for his extremely contested Cyber form. Also, Scorpion getting used to kill the ever loving daylights out of Sub-Zero constantly in promotional trailers to the undying ire of Sub-Zero fans, his borderline idiotic gullibility in the story, and his mere inclusion in Injustice finally cemented his status as this. Sub-Zero constantly getting the shaft in promotional works (which is almost all the time) has caused him to be seen as Scorpion's punching bag and a total loser, despite numerous evidence in-universe to the contrary.
It's safe to say that Scorpion might become this for the franchise on the whole. Signs of this began popping up in Injustice: Gods Among Us, when Scorpion was announced as a Guest Character and was met with almost universal scorn, especially when he was deemed overpowered before a patch fixed his attributes. Then, when Mortal Kombat X rolled around, fans began to notice a disturbing trend in the trailers (as well as those in games before this one) - any featuring both Scorpion and Sub-Zero will almost always end in Sub-Zero being completely beaten down and finished off in a gory fashion by Scorpion. Did we mention that he's currently the mascot for Netherealm Studios and is Ed Boon's favorite character? That being said, his status as an Ensemble Darkhorse complicates things and makes this very much an issue, as some fans liked him being in Injustice.
Game Breaker: Various characters at various points, which is why the series has never been particularly popular for Tournament Play. In some of the games, the bosses are available for use, but, keeping with a general rule for Fighting Games, playing as them is nothing more than an easy way to get people to stop playing with you.
"Friendship. Friendship again?". The "Friendship" one originates from a finishing move that karried through from MK2 to MK3 and Ultimate MK3. For "Friendship", the winning player would usually "spare" the losing player's life in exchange for some wacky kut-scene playing in the background. Thus, the announcer (Shao Kahn), upset that the player did not "finish him" as expected, would say the above phrase. Also, some of us will still say "Superb," or "Excellent", or "Outstanding" the way Shao Kahn says it in the games.
Also, the trend of ending words with "-ality", essentially turning anything in Real Life into a gory, MK-style finisher.
It's been clarified by John Tobias a long time ago (by the time of MK2, more precisely), that Raiden does speak nonsense in his torpedo move. They originally wanted him to yell something in Japanese, but they never found a phrase that sounded right.
Mortal Kombat 4 is considered as the game with multiple gibberishes cranked Up to Eleven. Also from the same game we got the ever campy "I'M SO GAAAAAAAY!" quote from Jarek in the endings he appears.
Nausea Fuel: It's a series famous for its gore. What did you expect?
Nostalgia Filter/They Changed It, Now It Sucks: Some say that it's as bad as the Sonic fandom, and that's saying something. This is especially evident in the borderline irrational hatred of 90% of everything that came out after Ultimate Mortal Kombat 3 and excluding Mortal Kombat 9. Case in point, many are already swearing off the franchise with the newest (and not yet released) installment Mortal Kombat X over some trivialshit. The reasons?
Vanilla Protagonist: In both moveset and storyline, Liu Kang was designed to be the simplest character. Up until Deadly Alliance, his main plotline was winning the Mortal Kombat tournament canonically and not much else.
Taven in Armageddon also fullifuls this trope. His main storyline is basically stopping the titular battle royale.
Adaptation Displacement: The theme song is one thing a lot of people know about the series, but it's never played in any games. It only appears in the movie.
The Sega CD version of the first game, released two years before the movie, had a version of the theme song on its opening. Both versions were available in a CD single sometime before the release of the movie. That's another layer of displacement.
Even the famous title scream gets this. It actually first popped up in the Mortal Monday commercials for the console ports of MK 1.
Awesome Music: Besides the main theme, there's the remix of said theme by Utah Saints, the instrumental version of "Control" that plays while Liu Kang and Reptile are fighting, "Halycon and On and On"... and that's just the tip of the iceberg.
Big Lipped Alligator Moment: The fight against Reptile, added to the movie in the last minute. Without explanation the CG critter Reptile fuses with a statue he was kicked inside to take an appearance similar as in the games, and have a long fight sequence with Liu Kang. It's never mentioned again.
Supposedly, it's because test audiences wanted more fights, so they went back to do some re-shooting. It's also why Johnny disappears during the course of the fight: Linden Ashby wasn't actually there for the additional filming.
More so however for Trevor Goddard as Kano. His performance was generally well received by fans and even the creators of the games loved his portayal so much Kano was actually changed from a Japanese-American with little character to an Australlian-born thug.
Christopher Lambert as Raiden was really entertaining, too.
Reptile too. Many whom seen the movie thought his fight with Liu Kang was actually the best fight in the whole movie; even better than the final fight!
Foe Yay: This can pretty much be seen as how Kano views his little rivalry with Sonya. His dialogue throughout the movie about her pretty much casts any doubt of this.
"She's here, right on time! I LOVE punctuality in a woman!" (smiles)
"Maybe Sonya and I should share a cabin, have our own little honeymoon cruise."
Shang Tsung to Sonya, big time. He's constantly flirting with her, complimenting her looks and saying he has "great plans" for her, and her Go-Go Enslavement look after being "challenged" by him speaks volumes.
Fridge Horror: After telling Johnny Cage about the tournament, "Master Boyd" is revealed to be Shang Tsung in disguise. Shang Tsung can only take the form of people he's defeated/killed in battle...
Genius Bonus: The traditional Kung Fu salute, palm over fist (well, it's palm against fist, but the palm overshadows) represents Peace over Power. Shang Tsung's odd bow before the final battle, fist on top of palm, is meant to represent Power over Peace, reflecting the opposing philosophies of the two characters.
Special Effect Failure: Instead of regular ninja-esque tabi, Scorpion can be seen wearing what look like hiking boots during his fight with Johnny Cage. And in a bit of Freeze Frame Failure, If you pause the movie while Reptile's doubled over from Liu Kang kicking him over a banister, you can make out a Nike swoosh on the soles of his feet.
Reptile's lizard form is a very obvious CG creature.
Tear Jerker: I can't be the only one that shed a tear when Liu said goodbye to Chan after defeating Shang Tsung.