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.
Demonic Spiders: Magikoopas - with their ability to cast homing projectiles, appear in inconvenient spots, and respawn - are one of the most dangerous enemies in the game. This is especially evident in ROM Hacks where they are inserted in Auto-Scrolling Levels and block-oriented puzzles.
Yoshi, the biggest Breakout Character aside from Luigi. His popularity led to him going from simply being a Power Up Mount to being an entire species and one of the most iconic characters of the entire franchise.
Chargin' Chucks, who only appeared in this game and Yoshi's Safari, but eventually proved popular enough to finally make a reappearance in Super Mario 3D World.
Funny Moments: Quite a few of the end-of-world cutscenes, but two stand out.
For Castle #4 (Ludwig's Castle), after Mario presses the plunger, the castle takes off like a rocket, and a confused Mario watches as it soars into the air and impacts in a nearby hillside – which wears a Pointless Band-Aid for the rest of the game.
For Castle #5 (Roy's Castle), when Mario presses the plunger, the explosives don't go off, so he goes over to inspect them... and they blow up in his face.
If you have the cape, you can fly over most levels. The same goes for the blue Yoshi, which can fly after eating any Koopa. Even worse, if you have a cape and a blue Yoshi (or any Yoshi and a blue shell), you can fly for a very long time without even having to mash the button.
The brokenness of the Cape extends to slowing down Mario's midair descent even more than either of the flight-oriented suits in Mario 3, making landings a cinch, and its spin attack kills most enemies and blocks most projectile attacks, making Mario near-impervious for any reasonably skilled player.
The blue Yoshi is broken to the point that, combined with the cape and a few timed jumps, it absolutely trivializes Tubular.
Unlike every following Mario game, you're allowed to keep any items that you have if you quit a previously completed level by pushing Start and then Select. This makes it possible not only to get out of falling into a Bottomless Pit, but to go into a level, get yourself a Yoshi and then load up and extra lives and power-ups, then go back to the map and rinse and repeat. Most subsequent Mario games would redress this by reducing you back to small Mario and stripping you of anything earned in the level other than extra lives when you quit the level.
Luigi in the GBA version is far more broken than in any other 2D Mario game. Combine the floatiness of his jumps from Super Mario Bros. 2 with the tighter control and higher jumps in general of this game, and platforming is completely trivialized even without the cape.
The Wiggler glitch in Forest of Illusion 1 makes Scoring Points ridiculously easy in the SNES version. Although the points cap is significantly lower, as there's one less digit allocated to the score display.
Magnum Opus: Miyamoto has gone on record saying that Mario World is his favorite Mario game, although he contradicted that in a later interview. Fans and critics generally agree that it's the best Mario sidescroller and a strong contender for the best Mario game in general.
Super Mario Western by Hyadain, a lyrical remix of World 1's theme featuring decidedly Gonk and skeevy depictions of Bowser and Mario, is often used to depict love triangles - love quadrangles if one counts poor Luigi, who's depicted as being a relative Bishōnen. There's also the inverse, turning everyone including Bowser into true Bishōnen and Peach is even cuter.
Throughout the game, Mario performs various methods of destroying castles after defeating the resident Koopaling. This has led to the "Mario's Castle Calamity" meme, wherein Mario tries to demolish a Nigh Invulnerable castle with methods that always blow up in his face.
Some people like the wonderfully dramatic, musical sting of the keyhole exits. Also, the pronounced stomping sound of jumping on a Chargin' Chuck is so recognizable that it was brought back for their return in Super Mario 3D World.
The sound the springs and note blocks make. BREE-E-E-E-E-E!!
In the SNES vs. Mega Drive/Genesis Console Wars, it was this game that received the full force of Sega's bile for being the direct competitor to the Sonic the Hedgehog series, and was constantly lambasted in all Sonic media (sometimes via Lawyer-Friendly Cameo) for being 'slow', 'boring', 'old-fashioned', 'childish', etc. etc.
It extended to some of the other systems as well. One commercial for the TurboGrafx 16 starts with "The passive type plays Nintendo" with Super Mario World shown (hilariously, the next part says "The aggressive type plays Sega" with a clip from Sonic the Hedgehog playing).
Sunken Ghost Ship, the final level before the Valley of Bowser. The section with Boos randomly appearing out of nowhere is quite frustrating, even when you realize that if you stay in one place, they can't get you.
Tubular (level 2) is a notorious example. The entire stage is basically making it across a giant pit with the P-Balloon Power-Up, except you need to actually stop and hit certain blocks to find more balloons, or else you're not gonna make it in time. On top of that, thanks to this, you're basically a One-Hit-Point Wonder, and you have to make it through fairly difficult attack patterns to hit said blocks. (Unless you can pull off the P-Balloon glitch (holding left and right at the same time, which is easy for ROMs but damn near impossible for an SNES D-Pad), which makes the level easy as cake.)
Awesome (level 4) is no slouch, either. A run across a slippery ice world with kicker Koopas sending shells at you every step of the way, followed by a platforming sequence over equally slippy narrow platforms with Banzai Bills, infinite flying fish, and finally a jump boost off a flying Koopa to another slippery platform to jump a decent distance to the exit pipe.
Outrageous (level 7) is a course that definitely lives up to its name, as Mario must run a relentless gauntlet of Wigglers, Amazing Flying Hammer Brothers, Jumping Piranha Plants, Bullet Bill cannons, pipes that require springboards to jump over (said springboards needing to be carried across several screens), and various other hazards.
Vindicated by History: While Super Mario World was always a big seller and well-loved, it was largely overshadowed by Sonic The Hedgehog 1 around their release dates since the latter had a much larger marketing push by Sega of America that lobbed endless insults at the game. Nowadays, it's a contender for best game in the series and one of the best games of all time. The main reason why is that Super Mario World was the first Mario platformer to heavily incorporate alternate paths that lead to alternate levels, creating a great sense of nonlinearity and choice. It only took time to discover all of the secrets.
The flavor text that appears after Mario defeats a Koopaling was originally uniform and didn't even list the specific Koopaling by name. Their names being mentioned as well as implied character traits (Wendy apparently liking to sing, Ludwig doing symphonies, and Iggy being demented) were added in by the NOA localizers.
Notably averted with the Koopalings' names. Super Mario World is the first game to give the Koopalings names in the Japanese version, and the names adopted were the same as the ones the games English localization created. Even then, the player has to stick around to the end of the end credits just to find them.
During Koopa's flashback in in that same episode, there is a Wheel of Fortune puzzle that is three letters in length (DUH). One year after this episode aired, the actual show would begin using three-letter puzzles in the Bonus Round.
Cheatsy's last line in "Gopher Bash" is "I hate those plumbers." Two years later, Dr. Robotnik would bellow a similar lament as his catchphrase: "I HATE THAT HEDGEHOG!"
Idiot Plot: "Fire Sale", "Send in the Clown" and "Ghosts 'R' Us".
Memetic Mutation: The entirety of the "Mama Luigi" episode, particularly "That's Mama Luigi to you, Mario! *wheeze*" Other notable quotes include: "Well, it's like they say in Brooklyn: Early to bed, early to catch the worm. Or, is it the bagel?", "Mario, wherever you are, HHEEEEEEELLLLLLPPPPPP!!!", and "Luigi, King Koopa has the princess locked up in his Coney Island Disco Palace!"
From "The Yoshi Shuffle": "It's a stone, Luigi; you didn't make it!"
Suspiciously Similar Song: Once again, each episode contains a musical montage. Except this time they used this from the start rather than using a cover song. For example, "Ghosts R' Us" uses a knockoff of Thriller, "Born to Ride" uses one of Born to be Wild, etc.