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.
Broken Base: The third world's dungeon is seen by some as a huge difficulty spike due to the shy guy pots and lots of spark platforms and increased length, while others see it as the easiest dungeon in the game due to the abundance of POW blocks, the near absence of instant death traps, the ease of escaping the Phanto, the fact you get all your hearts early, and the lack of Zerg Rush enemies in the dungeon.
Demonic Spiders: Bob-Ombs, when uprooted, often have an extremely short fuse, damaging you if you don't throw them in time.
Difficulty Spike: Depending on whether or not 3-3 is difficult for you or not, this either happens there or in 4-2, a level with a very hard to find fourth mushroom, Zerg Rush beezos BEFORE you get to the third mushroom, and some annoying platforming involving an autobomb, not to mention fighting Birdo on ice. The difficulty spikes again in 5-3 with an area overflowing with enemies plus a tough climb up inside a tree. Then you have to fight Clawgrip.
Fanon Discontinuity: To this day, there are fans who still refuse to accept the game as a part of the Mario series, never mind the fact that it was originally intended to be a Mario game anyway.
Peach's hovering ability. Don't have to worry about risky jumps jumping on disappearing logs when you her ability to jump and just levitate over them.
In general; Luigi breaks vertical jumping, Peach breaks horizontal jumping. (Mario's vertical jump height is actually better than Peach's.)
Peach and Luigi's super jumps, when used with the run button for great distance, can not only clear certain pitfalls with ease, but they can also bypass huge chunks of some levels to reach the end faster instead of going the longer normal way. Using said shortcuts will mean passing up a mushroom or two though.
Interestingly enough, the game manual declared Toad this, due to his speed and power, saying that you will use him the most, with Luigi and Peach only for levels that require their jumping abilities. It's probably worth noting that most Speed Runners do exactly this.
The Game Boy Advanceremake made shells rebound from walls instead of disappearing upon contact, and it now always generates hearts for every enemy it kills, making it ripe for exploitpotential.
Giant Shy Guys and Ninjis, who drop hearts when thrown, making them an easy source of hearts should you find yourself taking too much damage.
Never Live It Down: Its status as a palette swap of Doki Doki Panic ensures that DDP is almost always brought up in media that refers to this.
Sophomore Slump: Few people would call it a bad game, but where the original was the Trope Maker for the platformer genre and the next game is often ranked among the greatest games in history, 2 is mostly just considered to be a good game.
That One Boss: Clawgrip. He throws lightning-fast rocks that aren't always guaranteed to slow down enough before going down a pit. Good luck getting past him with Toad or Luigi.
Unwinnable by Insanity: In World 2-2 where you must dig through sand, it's possible to force yourself into a dead end by digging up every sand block until they're out of your jumping range. Since the game has no timer, you're stuck there. The only way out without resetting the game is to use the suicide code (Up+A+B on the second controller).
Serial Numbers Filed Off: Pirated versions of this game are sometimes called "Super Wonderful Mario" (on the label) and "Super Bros. 5" (on the title screen). The game is completely the same.
Vindicated by History: In 2012, Shigeru Miyamoto shocked many by dubbing this as his favorite Mario game. General Audience reaction has been kinder towards the game in more recent years, partly because it is the game that most arguably breaks the formula and introduced numerous defining gameplay mechanics.