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.
Big In Japan: This game is much more popular in Japan than in its home country of the United States. In fact, there are several Japan-made fangames, such as I Wanna Be the Love Trap. Videos of this game are popular on Nico Nico Douga, the Japanese counterpart to YouTube, and get much more views there than on YouTube. Also, many Japanese players have recorded themselves playing the game on Impossible (no verified full clear run has been made without autofire, however, with the exception of two which are both hosted on YouTube).
Breather Boss: The Dragon Devil is a very simple boss once you master the jumping at the beginning and ending. Of course, it's still difficult, but it's far from being the hardest boss.
Crosses the Line Twice: The game is so insanely over-the-top difficult that you will change your reaction to dying repeatedly in the same spot from annoyance to cursing to laughing. It's the only way to handle a game like this.note Of course, some people head back to cursing again after a while. It can get a little, well, messy.
Alternatively, it's many players' introduction to Platform Hell games, particularly non-ROM Hack games of the genre.
Goddamned Bats: Most of the enemies. Strangely enough, the bats are probably the only things in the game that actually don't kill you upon touch.
Goddamned Boss: The Koopa Clown Car. There are three different pilots (Bowser, Wart, and Dr. Wily), and you need to beat all three in a row. Bowser and Wart are long, yet incredibly easy, and the real fight is against Wily. However, every time you die to Wily, you need to do the whole thing all over.
Good Bad Bugs: Some bugs that benefit the player were actually kept in the game.
What may very well be the most game-breaking "bug" is the fake save point on Impossible. Killing it makes a perfectly usable save point appear for exactly one frame on the screen. Killing it while having a bullet exactly where the save point would appear allows you to save the game.
Some bugs allow so much sequence breaking that people have completed the entire game in 18 minutes.
In the Tetris room, it's actually possible to jump through some of the pieces. However, the jumps are quite difficult to time, so it's not really that useful.
In the fight with Kraidgief, it's possible to avoid the entire fight by repeatedly pressing the 'K' key until he dies.
Inferred Holocaust: The Kid's grandfather was killed by The Kid's father to be The Guy. The Kid's father was killed by The Kid to be The Guy. Take a guess what might happen to The Kid when he has a son. Doubles as Big Screwed-Up Family.
Special Effect Failure: The fake error message in the Castle of the Guy is kind of hard to fall for if you aren't running Windows XP—which is outdated and no longer supported as of 2015—and using the default theme.
That One Boss: While all of them are plenty hard, Dracula takes the cake as a result of being the only boss without a set pattern.