A.I. Roulette: Dracula takes this and turns it into something from the tenth circle of hell. His randomly picked attacks range from standard difficulty (i.e. hard) to beyond good and evil, and some combinations are pretty much unsurvivable. This makes him the potentially most difficult boss fight in the game (yes, he can be tougher than the Final Boss).
It would sound incredibly silly used in any other context. In this context, though, its badass-level is somewhere between single-handedly crushing the universe into paste and ripping out someone's intestinal tract and using it as a belt.
"I defeated The Guy" works well in real life, when you're among people who are familiar with the game. The higher the difficulty, the more badass the boast.
Bait-and-Switch Boss: Near the end, the Moon that has been plaguing you appears all huge, as if ready to engage in a final showdown, and then gets smacked aside in about two seconds by the dragon from Mega Man 2. Especially misleading in that the Moon was high in the sky for the entire minecart ride beforehand, and the minecart section had "The Moon" from DuckTales playing as the background music.
In fact, the Moon does the same thing much, much earlier: throughout the Ghosts 'n Goblins area, the Moon appears bigger and bigger with each screen. On the screen with a Full Moon, you head under it and the Moon falls down onto you. Instead of killing you, of course, it just breaks the floor and drops you into the Mecha Birdo boss fight.
The I Wanna Be the Guy Mugen fangame has you fight many of the bosses from the original game. One of the bosses starts with the moon, which again gets knocked aside by the Mega Man dragon. Then the moon reappears and smashes the dragon!
To The Bat Noun: Each room in the final dungeon has "—of The Guy" tacked onto its name.
Big "NO!": The Guy, once he's well and truly dead.
Bilingual Bonus: The Japanese on the mock-Ikaruga warning before Mecha-birdo says "I hate Dorayaki". Incidentally, it's also misspelled with the character for "jealousy" where it should say "fried", perhaps to match the quality of the English text.
BFG: The Kid claims one as a result of triumphing over The Guy.
Boss Warning Siren: The game, parodying Ikaruga, provides a warning when Mecha Birdo is entering the room.
Brick Joke: In one screen you can clearly see the moon. Several later, it falls and tries to flatten you.
Bullet Hell: The game tries this a few times. Dracula has an attack where he shoots a spiral pattern of flaming Delicious Fruit; it's no Touhou pattern, though, as Kayin admits. Later on, the Vic Viper sequence shows Dracula how it's really done.
Also, the Tourian section plays with this in the boss room. Of course, it's really more like Rinka Hell.
The moon falls out of the sky multiple times during the game. Dracula shoots it at you, complete with voice-acted "Here is TRUE POWER!" and a trail of fire.
Surprisingly, the drops themselves aren't likely to kill you. Instead, the primary cause of death via moon is it chasing you. In fact, the one time it only falls directly down, it actually doesn't kill you. This happens because it crushes the ground beneath you, sending you to the Mecha-Birdo battle
Cranium Ride: On 8-bit Link in one Zelda-themed room. Link attempts to swipe at you in the process.
Deadly Dodging: The Guy's third attack pattern renders him Immune to Bullets. You gotta bounce his shots back at him. Which is not easy, because instead of obeying rules of motion or bouncing at proper angles, the shots always aim at you when they hit a wall.
It's so common that it feels a lot worse, especially since every part of every room is a death trap. Even the things that aren't dangerous at all catch you because you're expecting something awful to happen.
There are six hidden rooms with secret collectible items. One requires going through two screens full of small and completely invisible platforms in the wrong direction. And mind you, that one is the easiest one to get. Don't even ask about some of the others...
Wait long enough at the title screen. See what happens.
Easy-Mode Mockery: Few people wanted to play the game on Medium because the extra save points were marked "Wuss" and The Kid gains a cute red bow in his hair... The bow remains on the screen when you explode, too.
Fake Difficulty: This is sort of the point of the game. You'll spend a lot of time with the trial and error twins. That being said, the examples of this trope are all spaced out by examples of very, very real difficulty, so that even returning players will have huge troubles.
Fission Mailed: The fake Windows XP error message, which drops down and squashes The Kid if you don't move him out of the way in time immediately after regaining control. Especially ironic since the game crashes so much that there's a decent chance of getting a real error message instead.
The Kid falling onto a giant bed of spikes at the end. As he has defeated The Guy and claimed his title, he walks out unharmed.
There's a level which certainly deserves the Fan Nickname "The Room of Huge Laser Beams That Come Out Of Absolutely Nowhere", based on Quick Man's infamously hard stage from Mega Man 2. And frankly, it might actually be easier than the original.
Gotta Kill Em All: In keeping with the Metroidvania theme, you need to take out six bosses before you can enter The Guy's estate: Tyson, Bowser, Dracula, Mother Brain, Kraidgief, and Mecha Birdo. The gateway is a shout out to Super Metroid, specifically the entrance to Tourian which is inaccessible until you remove the statue depicting the game's bosses (which won't budge until they're all beaten). Like in Super Metroid, you can tackle the bosses in any order.
Mecha Birdo's third weakpoint shoot at its mouth as its firing an egg is something you're probably not going to know about even after you do find it.
One of the lefthand walls in the starting screen is fake and you can pass through it, but the only in-game clue is that your bullets pass through it instead of disappearing. And considering this is your first experience with the game's many death traps, you're probably not going to notice the clue — you're more likely to stumble on it by complete accident, or know about it already from watching playthroughs.
Harder Than Hard: The difficulties start at "Medium" and go up through "Hard", "Very Hard", up to "Impossible".
Note that the actual game itself is always the same no matter what level you're on. The only difference is the frequency of save points (and the Easy-Mode Mockery on Medium). There are zero real save points on Impossible. You're expected to win the entire game with one life. In all literal seriousness, Impossible mode is possible, but requires quite a bit of memorization and a fair degree of benevolence from the more random luck-requiring bosses. When somebody eventually beat it, the creator's official response was "holy crap you're not serious are you".
Homage: Pretty much everything in the game is a recreation of or reference to some old famous game, including Tetris, Kirby's Dream Land, Ghosts 'n Goblins, Mega Man, Metroid, Castlevania, The Legend of Zelda, and more. Especially the harder parts. Most of the bosses are actually taken from other games, though made much harder.
Let's Play: This game was the first LP ever done by Rawrzaur, all the way back on his previous channel of Zero Megaman X. He has gone on record during one of his live streams as saying that the original reason he even started LPing in the first place was to prove that he could actually beat this game.
Long Song, Short Scene: There's a ton of great 8-bit music in this game. The problem is that, with death so frequent, you'll inevitably only hear the first few bars on each life.
Lost Forever: If you don't grab the hidden item in the Metroid level the first time around, you can't get it on that play file, because the Metroid level is inaccessible after the Tourian System explodes.
Make My Monster Grow: The Guy starts out as the smallest boss in the game (but still notably bigger than The Kid). He does this to himself partway through the battle.
Malevolent Architecture: You will most likely find yourself getting killed more often by the environment than by an actual living enemy.
Masochist's Meal: People eat Delicious Fruit. According to Kayin, they have to get them out of trees with sticks, in order to keep from careening around for miles and killing everyone, and then they have to boil them three times to remove all the poison. The bouncing fruity engine of death seen in the Breakout section is what happens when you only boil them twice. Delicious!
Ninja Prop: Obvious parts of the background attack the Kid.
Nintendo Hard: Ironically, in order to make the game beatable, you have infinite lives and there are even save points. This is an almost unprecedented level of accommodation in the sidescrolling genre, but it's justified since, well, it really would be (almost) impossible without them (which is indeed exactly what Impossible mode does).
To put this in perspective, Mike Tyson, one of the most difficult final bosses in video game history, is probably the easiest boss in IWBTG.
One Bullet at a Time: In most parts, you can only have four bullets on the screen at a time, making it important to get as close to the bosses as possible.
One-Hit-Point Wonder: Contact with most moving things (as well as several things that don't move) that are not platforms is instant death. If it doesn't outright kill you, it will likely bump you towards something that will.
One-Winged Angel: The Guy plummets out the window after you shoot him full of holes... Only to return as a giant head.
Save Scumming: It's almost impossible to complete the game without ever saving. Especially not on your first try.
Selective Gravity: A rule of thumb is: if something can inconvenience you by falling upwards/sideways, it will do so.
Sequential Boss: The Koopa Clown Car. Unfortunately, the first two forms are so easy, it's basically just an unskippable 2 minute cutscene every time you want to fight the much harder final form.
The Guy has two major forms, both divided into three phases. While these are the only two bosses with different major parts, every other boss except Mother Brain has multiple phases. Well, except for Dracula, who parodies the trope.
Stalactite Spite: Most commonly done with fruits or spikes (see above), but there are some genuine stalactites in there too. Subverted at one point with ceiling spikes that tremble when you pass underneath them, but never fall.
Stand-In Portrait: Inverted with the Vic Viper, which ends up being very useful to The Kid.
Stepford Smiler: Whether The Kid traverses through spike-covered corridors or tries to avoid gravity-defying fruit for the billionth time, he always does so with a smile.
Stylistic Suck: The magnified Nintendo sprites (Mike Tyson, Mecha Birdo, Kraidgief, and The Guy) look appropriately ridiculous.
The Tetris Effect: Don't get surprised if you start expecting traps and surprise deaths in every other game you play. Or hell, on your way back from school! Or in your lunch box, next to your sandwich!
The Unfought: The Moon despite what you may think upon reaching the first boss of The Guy's castle.
Throw It In: Word of God says the Kraidgief bug was kept in "since it works just like kraid from Super Metroid".
Trial-and-Error Gameplay: If you look hard enough, you might be able to find a room that doesn't do this. It's worth noting that the room seen in the screenshot at the top of this article is one of only about four or so in the entire game that lack out-of-nowhere surprises and are Exactly What It Says on the Tin. It's anything but boring.
Unskippable Cutscenes: Thankfully averted. If you don't want to go nuts from having to watch Mike Tyson rise up every time you die, remember, the "S" key is your friend. Perhaps just as well, considering Dracula can kill you with his goblet during the cutscene before the fight with him.
However, the lengthy intros to the Kraidgief and Bowser fights can't be skipped. For the former, this is actually good, as you can shoot his head for a bit once he roars, which wouldn't happen if you skipped it. For the latter, this is due to it being a three-part boss.
Unsound Effect: In the Clown Copter battle, when you beat the first two pilots, the Copter spins off with the cartoon explosion reading "Bomb!" from Super Mario Bros. 2.
Unwinnable: This can occur at many parts of the game where if you fall down and can't back up or something of the similar sort, you can't progress. Luckily, the game has a suicide button... which does you no good on Impossible, which screws you over for trying to restart by wiping all your progress on the run. Also, the game makes no attempt to prevent you from saving in bad places, meaning that it is easy to unintentionally overwrite your save with an Unwinnable situation. Thanks Kayin for I Wanna Be The Fix.