The Kid. All he can do is double jump, something that Ogmo, a character that is easier to unlock, can already do. What The Kid is important for is bragging rights; if you manage to unlock him, you're essentially better than 98% of all Super Meat Boy players. However, the Kid is faster and a bit lighter than Ogmo, which lets him do tricks that Ogmo otherwise can't do.
Also, Ogmo's double jump is more horizontal than vertical while The Kid's double jump has pretty good height.
Butt Monkey: Both Meat Boy and Bandage Girl get abused quite badly.
Dr. Fetus gets this too, especially given the intro. "Nobody likes Dr. Fetus, and that's why Dr. Fetus hates you!"
Effectively, every single character that appears in a cutscene is one of these. Especially the tragic squirrel, who is the lone survivor of the forest fire, is attacked by a rogue flying buzzsaw, is vaporized by a nuclear explosion, and STILL manages to come back at the ending cutscene, totally beaten up. Most bosses are also killed off in a rather dark-humored way. They all have a happy ending in the final cutscene though.
Flywrench. It takes a while to get used to its controls due to being very heavy and unruly. However, it can do a triple jump and scale up walls without having to jump off them which will become very handy in some of the levels.
Naija is another of those characters who seems to be difficult to control at first, but becomes useful when practiced with.
Level Editor: Meat Boy featured one. Super Meat Boy gets one for the PC version. A prototype devmode was accidentally left in the game which resulted the creation of user-generated content much earlier on.
Malevolent Architecture: Who puts buzzsaws in salt factories and skyscrapers anyway? (At least the forest sort-of made sense as an out-of-control logging operation.)
As you play through the Hospital, you'll begin to wonder where all these syringes come from, and why they're stuck to the walls and ceiling, needle-down.
Serial Escalation: Take a shot every time you think it can't get harder and are subsequently proven wrong. Continue until you're too wasted to play.
Shout-Out: The game has a large amount of them, which is expected from an indie game.
The Flash game opens with Dr. Fetus stealing Bandaid Girl ala Ghosts N Goblins. This is also present in Chapter 7 of Super Meat Boy, except Dr. Fetus captures Meat Boy instead. Super Meat Boy likewise begins each chapter with a re-enactment of the opening to a different game:
Advancing Wall of Doom: At least a few times per world. They take the place of the autoscrolling levels in the original game. The "wall" may consist of saws, blood, salt, lava, maggots, or, in one of The Cotton Alley's stages, Pepto-Bismol. (It's a palette swap of the lava.)
C.H.A.D. and Brownie have a rising death floor as hazards in their stages.
The respawn after death literally take less than a second and is automatic.
The levels themselves are short, ranging from 15 seconds to 90 seconds, so in the quite likely event you die, you don't have to go through too much again.
Various little side-quests when you get too frustrated with the main game, like beating past levels in record time, collecting bandages to unlock new playable characters, or playing through retro-styled "warp zones".
When you finally do beat a level, the game then shows you a replay of all your past lives doing the level simultaneously, which is good for showing you where the hardest parts of the level were. It's also kinda hilarious to see a cloud of Meat Boys get shredded to half their number by a giant saw.
Cranial Eruption: The 4th boss gets two of these during the fight. Since he's made out of dead meat, they're rather blocky.
Crapsack World / Crapsaccharine World: If you aren't dead, you're about to be, and everything kinda sucks while you're alive anyway. A squirrel can witness his whole family die, only to be killed by a random saw blade. He gets better, but only in time to be killed by a nuclear explosion to get a bit better again during the ending. Heck, even Dr. Fetus is only evil because everyone hates him! Also, most of the locations Meat Boy visits seem to be in ruins.
Creator Cameo: Complete one of the minus worlds and you see the creators of the game make an appearance.
Crutch Character: Commander Video, sort of. His float makes it easy to clear levels and get bandages in the first two chapters, but he's very slow and his jump is one of the lowest in the game. Many levels later on are impossible to clear with him due to his flaws. However, it's still somewhat possible to get some bandages easier in later levels.
Cut Song: The Dark World of Chapter 5 has an unused theme. Same to second part of the boss level.
Dark World: Each of the main levels except the boss battles (not including Chapter 6's boss) has a dark world counterpart. You don't know why the other ones are called light worlds until you play these.
Deadly Dodging: A few times it has to be done. For an example, to allow turrets open up the passages. The fifth boss fight requires you to lure the boss to his death.
Deadly Rotary Fan: Several levels have giant fans that can propel you through the air. Get too close to the blades, though, and SPLORCH!
Disney Villain Death: Subverted — Dr. Fetus appears to have died after being defeated Super Mario Bros.. style in the final boss level, but then he pulls off a Taking You with Me by activating a self-destruct. After Meat Boy and Bandage Girl escape, Dr. Fetus reappears and hits Bandage Girl over the head repeatedly.
Double Subverted — He actually accomplishes nothing by hitting her, which he soon realizes and falls to the ground. Then he's stomped on. A lot.
Distressed Dude: Eventually, Dr. Fetus decides that he has had enough of capturing the girl and captures Meat Boy instead.
Doppelgänger: Meat Boy has four and counting. And that's just playable characters. There's Brownie, who's a traditional Dark Reflection type doppelganger, Tofu Boy, pretender to the throne turned pathetic Joke Character and star of PETA's ill advised parody game, Meat Ninja, Meat Boy as a Future Badass from the year 20XX, and most recently, Potato Boy, an April Fools joke and Portal 2 tie in who's afraid of girls. Not to mention the fact every time Meat Boy dies, his former body lives on as a Meat Boy zombie as seen in Hell and Rapture, of which there are doubtlessly thousands. Then if that weren't enough, the Meat Boy zombies can merge together to form a colossal Meat Boy golem called "Little Horn", which serves as the game's fourth boss. Despite all this, there is not yet a Robo-Meat Boy.
Double Jump: Some of the unlockable characters can do this, and one can even triple jump.
In the Xbox 360 version, there's an extra chapter called "Teh Internets" (unlocked by collecting 20 bandages) that has 5 sets of 20 levels.
Dummied Out: In the PC version, there's a primitive level editor, a complete character select, and a playable Dr. Fetus, complete with functioning rocket launcher.
The End... Or Is It?: The complete ending shows that Bandage Girl is pregnant with an evil fetus. Dun dun duuuuuun!
Everything's Better with Rainbows: If, for some reason, you don't like playing as Meat Boy and dripping a trail of blood everywhere, one of the first characters you can unlock is Commander Video from the BIT.TRIP series, who has an 8-bit rainbow trail.
Evil Knockoff: Brownie, who is designed to look like Meat Boy, but instead of meat is made of Dr. Fetus' poo.
In one of the early levels, it's possible to get bandages over and over, resulting an incorrect percentage on the game completion. It also had a save file crash problem when it came out. The bugfix is still awaiting approval.
When the PC port came out, many players also had some serious bugs ranging from bosses not appearing to not being able to start the game at all. The development team started to fiercely fix the glitches so it got 9 batches of bug fixes within 9 days.
Unfortunately, the use of variable framerate physics makes the game so glitchy that it is unplayable on performance-challenged computers.
Heh Heh, You Said X: If you Alt+Tab while playing, you will return to your computer's desktop with the game minimized. If you try to bring it back up again, you will sometimes get an error message stating that "The game has crashed. A mini-dump (heh heh) has been sent to the developers."
Homing Projectile: A few of the turrets fire these. Although their homing abilities aren't the best, they make up for it with projectile speed.
Also, Dr. Fetus' bazooka. While its projectiles are slower and smaller, their homing abilities are much better.
Meaningless Lives: Averted in the warp zones but played straight in the Minus Levels; since you only play one long stage at a time (as opposed to three in a warp zone), lives seem to serve no purpose other than to annoy the player with loading screens.
Medium Blending: The creators wanted to give art shifts opposite of Ren and Stimpy (where characters and everything else get extremely detailed in closeups) so in-game graphics, where everything is distant, are drawn more realistically, but menus and cutscenes are drawn in cartoon style. In-game characters, however, are drawn in a very pixellated style.
Minus World: Deliberately used. Bandage Girl starts glitching sometimes when you replay levels in a chapter where you've beaten the boss. Reaching her when she's in this state unlocks that chapter's Minus World.
Mood Dissonance: Cotton Alley. The background is pink (very bright in Dark World), a rabbit disco plays in the background, and the chapter contains some of the hardest levels in the game.
Not-So-Harmless Villain: At first, Dr. Fetus seems like a comical Jerkass villain whose worst crime is rather slapstick abuse of Bandage Girl. Then, at the end of the first world, he burns down the forest the earlier levels took place in For the Evulz and chases Meat Boy down in a giant saw robot powered by the blood of innocent woodland creatures, and it gets worse from there.
Oh Crap: Meat Boy's reaction when he has a bazooka in his face.
In 3-11X, one of the harder Salt Factory stages, it's possible to jump between or above the advancing buzzsaws so you don't have to go all the way to the right to jump over saws.
The Kid and Flywrench can become handy in some of the later chapter levels due to fact that in some levels, he can often skip the portions of the level entirely.
In the final boss level the sawblades advancing in front of you can be jumped over for additional speed. Oddly, if you get to your destination before the blades in that level, you become mysteriously immune to them.
Single Tear: Happens in the cutscene after beating the first boss.
Spikes Of Doom: Although they're replaced by syringes, salt and so on in most of the levels, genuine ones appear in some of the warp zone levels. In the Hell and The End chapters, some very sharp spikes are encountered.
Why Don't You Just Shoot Him?: Dr. Fetus finally shoots Meat Boy three times with a rocket launcher in the last level. He respawns three times, just like he does whenever he get shot, cut, or otherwise killed.
To drive this home even further, in Cotton Alley, where you play as Bandage Girl and you have to save Meat Boy, whenever Dr. Fetus shows up to disrupt your reunion, he never beats Meat Boy up before abducting him.