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.
Alternative Character Interpretation: Most of the rappers' personalities are exaggerated from what history remembers, or canon in the case of fictional characters. Example, Bieber is depicted as an annoying, pint-sized brat, but more likely than not, the real Bieber is not as annoying (some of the time). Mister Rogers was normally a friendly man, but here he is a vicious yet soft spoken sadist, although if only because Mr. T accused him of being a pedophile.
Broken Base: Batman vs. Sherlock Holmes is possibly the most polarizing episode yet. Half the fans like it just fine, the other half found Batman's delivery of his verses to be off-putting, on top of Batman having very few good lines.
Some comments in the Gandhi VS MLK battle suggest that more than a few people didn't find the portrayal of either all that honorable towards the men.
Rasputin vs Stalin disappointed some of the viewers.
Crack is Cheaper: Bill Gates says that people can still afford to get medical help if they buy a PC over a Mac.
Dude, Not Funny!: Using Marilyn Monroe's multiple miscarriages and her death by barbiturate overdose as "disses" in "Cleopatra vs. Marilyn Monroe". Obviously, rap battles aren't expected to be civil, but still.
A couple of Hitler's lines come off as this to some.
Clint Eastwood's lines about Bruce Lee's death by aspirin and the death of his son were seen as low by some.
Sherlock Holmes might have taken it a bit far with his insults about Batman's parents' murder. That said, he was reacting to Batman's drug abuse/Dying Alone comment.
Frank Sinatra's making fun of Freddie Mercury's AIDS could also come across as this.
While Hitler is at least sort of Laughably Evil, Joseph Stalin here is just as horrifically unstable as the real Joseph Stalin.
Ear Worm: You will never, ever get Steve Jobs vs Bill Gates out of your head.
"Funny Aneurysm" Moment: Rap Battle #5; The Macho Man died about 4 months later after it was released. Then Kim Jong-Il passed away in December, though Kim could be considered an Acceptable Target, much like Hitler.
Happens constantly during "Einstein vs. Hawking", obviously. For example, p-brane (pronounced the same as pea-brain) is a vital element in string theory. Also, 'relatively blown' refers to Einstein's best known theory (relativity).
The entirety of Shakespeare's first verse is in iambic pentameter. Minds will be blown.
Not only that, but it's complete with Ye Olde Englishe phrases, pronunciation and theatrical gestures.
Said to the Wright Brothers: "You might fly like a hawk, but you fight like a kitty!"
Bill Gates VS Steve Jobs is full of tech-based jokes.
There's a delicious double pun in the line "I think different from the engine of the days of old" - simultaneously playing on Apple's old slogan and Charles Babbage's original computer.
Mozart vs. Skrillex is full of awesome music in-jokes (i.e., "I attack! You decay!")
Vader off-handedly referring to the Stormtrooper clones being superior to Hitler's.
Josef Stalin's right hand was reportedly withered, which is directly referenced by Lenin in the Season 2 finale.
In the same video, Vader moonwalks. He was played by NicePeter, as was the adult Michael Jackson in season two.
In the rematch, Vader comments that he's "on the leader of your limp-dicked Luftwaffe" before making a reference to Lucas's other famous production, Indiana Jones. Then another Lucas production about pilots taking down the Luftwaffe came out.
Lady Gaga makes a reference to the Jackson 5. 15 episodes later, The King of Pop himself (with a short appearance by his sibs) takes a shot at the mic. Both Gaga and Adult!MJ are played by Nice Peter to boot.
Speaking of which, what about in Battle 6 when Justin Bieber says he's the next Michael Jackson? Or Genghis Khan telling the Easter Bunny that he's about to feel Wrath of Khan before Captain Kirk rapped?
Stephen Hawking proclaims that he's the "Snoop Dogg of Science". Snoop Dogg himself would end up playing Moses 20 episodes later.
In the trailer for Season 2 at the end of Episode 15, a quote from Vladimir Putin appears, saying he will kill Nice Peter if Season 2 does not have a Russian character. The Season 2 finale has five Russian characters, of which one is Putin.
Most Annoying Sound: A bizarre example as it doesn't come from the Epic Rap Battles at all. After "Bill Gates vs Steve Jobs" battle came out, many fans went looking for an instrumental version of the beat used, only to quickly grow tired of the original's use of "Tristan on the track" as a refrain. Tellingly, numerous versions now exist on youtube, editing it out.
Darth Vader in the rematch when he removes his helmet.
HAL 9000 intrudes on the battle between Bill Gates and Steve Jobs. He's appropriately eerie, boasting of his superiority in that infamous monotone of his. The most terrifying thing about him is that he does not seem to move on his own, nor does he emote.
After Steve Jobs dies in his battle with Bill Gates, Gates seems to go insane, especially compared to his previous Tranquil Fury... just look at his last verse:
Now there's no one that can stop me! THE WORLD IS MINE!
The especially creepy part is how he seems to realize this mid-verse, going from yelling at Steve's death to "I'm alone now, with nothing but power and time...and no one on Earth who can challenge my mind!" His sudden change is frightening.
Skrillex briefly turns into a "Scary Monster" that looks kind of creepy.
Stalin's portrayal is quite...unsettling. Most of his lines are part of a long, thorough death threat towards Rasputin and everyone he knows. What makes this worse was that he wasn't any better in Real Life.
YMMV, because that whole rant made it on the Funny Moments page, too.
Thing 1 and Thing 2's high pitched voices, creepy look and Ax Crazy lines make them scary.
Painful Rhyme: Being based on rap, this was unfortunately inevitable.
"Look at these two, their lives must've been horrible / Two dorky dudes named Wilbur and Or-i-ville"
Rhyming "streets" with "mysteries" in Batman vs. Sherlock Holmes.
Einstein rhyming "serious" with "theory was"
Lenin rhymes 'Right hand man' with... 'Right hand, man.' The context it's used in is clever, but it's still unpleasant to the ear.
Thing One and Thing Two aren't exactly popular with the general public.
Thomas Edison got only ~15% of the votes on the official page. Just like Bieber's case, this was probably an Intended Audience Reaction.
Probably nothin', all the rap battles tend to depict one side more negatively. Hint: it's usually Lloyd.
Win Back The Crowd: The fandom was displeased with the unannounced hiatus that happened after the Bill Gates VS Steve Jobs, some even going so far as to thinking that the work would become an Orphaned Series. Cue the announcement that the next six Rap Battles would be uploaded back-to-back every fortnight, followed by a holiday break and more episodes. This was impeded only by a small Schedule Slip in regards to Batman VS Sherlock Holmes.
The Woobie: The Easter Bunny really bit off more than he could chew.
Freddie Mercury starts off as one. He spends most of Frank Sinatra's first verse and part of his first verse moping at his piano. Then he gets going.
Iron Woobie: It's subtle, but Vladimir Lenin apparently watched his most trusted student dismantle his efforts toward a socialist paradise in favor of a brutal dictatorship. The reason you don't notice is that Lenin vents about this with authority.