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.
YMMV: Super Mario Bros X
Game Breaker: The Hammer Suit is back from Super Mario Bros. 3, and once again can kill nearly everything. Including Thwomps, Boos, and Podoboos. You can even modify the angle of the throw!
Yoshi is back from Super Mario World, and he's just as useful here as he was there. The original 4 colors return and are joined by new varieties, including a purple Yoshi that can Ground PoundYoshi's Island-style and a pink Yoshi that turns everything it eats into veggies (which it can spit back out). However, Yoshi with a blue shell (or blue Yoshi with any shell) has been nerfed, and is now merely about as flight-capable as Raccoon Mario - he can't keep gaining height forever. Considering that Raccoon Mario's flight is somewhat game-breaking in its own right, this is less severe than you might think.
In early versions, Yoshi with a blue shell actually could fly forever. Game Breaker indeed.
The in-level items might also count, such as the Billy Gun (shoots bullets at an extremely fast pace), Toothy (a Piranha Plant in a pipe that kills enemies for you) and most of all the Super Flamethrower (which combines the Propeller Block to slow your fall and fast-moving Yoshi fireballs). However, you can't bring these out of the level they're found in, and there tends to be a proportionate amount of enemies in the area.
To elaborate, green Spiny eggs CHASE YOU, and they almost always have strength in numbers because the only default Mario 3 Lakitu throws them.
Thankfully, a spin jump will dispatch them. (Bug, maybe)?
Rinkas. As in the energy rings that continually shoot at you, straight from the Metroid series. The worst thing about them being that you cannot stop them from coming until you run away.
There's also the flying enemies coded to track you. What makes them particularly annoying is they track you in an undefined, roundabout way, making it difficult to predict where they'll come at you.
That One Boss: Mother Brain in the Secret Star Exit of Pwnhammer Duece. It wouldn't be so bad if it wasn't that your only weapon for the battle is a rainbow-flashing Koopa Shell, unless you have a hammer or Link's sword, and the fact that there's blocks shooting harmful rings at you, ALL THE TIME. And she has 12 hit points.
That One Level: Hardmode Tanks. More tanks, more enemies, and MORE Birdos to defeat. They spit... fireballs. All the time. You'll need a hammer suit for this one.
The Secret Star Exit of Pyramid of Fail. In order to get it, you have to go PAST the exit. Indication that this exit is hard is that there seems to be no floor... when there is safe ground above lava, whereas it's INVISIBLE. Only Tanooki Mario would be able to reveal the path correctly by transforming into a statue.
Also qualifying is the Secret Star Exit of Dire Dire Rocks. You have to collect all fifteen red coins. In areas filled with Cheep Cheeps. And if you die, start the search all over again... and again... AND AGAIN...
In the episode Return to Yoshi's Island, we get the Day at the Races levels. The first one, which is only the second level in the entire game, isn't actually that bad, but that's only for the normal exit. The hidden star in this level is accessed by racing the Koopa the Slow. Sounds easy? note (He's named Koopa the Slow, so he's slow). Too bad it's not Koopa the Slow who's racing you. It's his shell. And it goes faaaassstttt. You have to have near perfect timing on all tricky parts if want to win, plus if you DO lose, you have to kill yourself if you want to start over again.
Day at the Races 2 had another hidden star, and it's arguably even harder than the first Day at the Races. This star can only be accessed by playing in 2-player mode, and in this Koopa the Slow's shell race, you have to ride on a rainbow shell to get there. It's almost impossible to win. Trying to place the door exit to the finish line so you'll win easily? Won't work. Yes, a random Layer/Event made it impossible for you to win when placed near the finish line. Yeeeesh.