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.
Designated Villain: Excluding the first battle in the game, every time Bowser fights the Mario Bros. he's technically not doing anything wrong since he's been struck with amnesia and is either simply following Popple's orders or possessed by Cackletta.
Popple will be taking this status, too, see? He even made a reappearance in Dream Team, albeit only for a bit part.
Evil Is Sexy: Averted with Cackletta, who is the first real female villain in the franchise to do so.
Game Breaker: A badge that increases the Brothers' power depending on how many mushrooms they have. Go ahead and buy 99 of each mushroom. No one will stop you. This also makes Great Force more usable. Great Force doubles the damage you deal while also doubling the damage you take; that second part doesn't matter so much because, with both Great Force and the Mush Badge, you can destroy almost every boss in one round.
Get Great Force ASAP, which is after Chucklehuck Woods. Have one Brother equip that and have one equip the Bonus Ring, which you must have if you have Great Force. If you fight every enemy in the game after and take little damage, you can one hit kill almost any enemy in the game. Bosses can get taken down in two turns. With the Bonus Ring, which doubles Experience if you don't get hit, Great Force's side effect of the Bros. taking double damage starts meaning less, especially with bosses, where there are Save Albums right before... needless to say, a Great Force-Bonus Ring combo destroys anything, enough to the point where it's plausible to take out late-game bosses without Bros. attacks, including the final boss. Not as game breaking as the above example, but still makes the game pathetically easy.
The matching pants that complete the ensemble—the ones that boost your luck (chance to crit for extra damage) based on your Mushroom count.
Get the Advance Thunder Bros. If you get it right, then it reduces the enemy's attack so that they only do 1 damage with each hit. And, it works on most bosses! It only hits one enemy, but since many of the bosses have only one section, that frequently doesn't matter.
The easiest way to max out all of your stats is to drink nothing but Teeheespressos. This is because Woo Beans and Hee Beans are the easiest beans to collect, REGARDLESS of the fact that you can't specifically choose which stat you want to raise with Teeheespressos.
Remember the entry about the Mushroom vs. Beanbean exchange rate? Now consider that Mario & Luigi: Dream Team ends with the Zeekeeper showering 80 million coins on Pi'illo Island, which would be used as a tourist attraction. Guess the big bird didn't really take inflation into account....
Ho Yay: Luigi is flattered by Prince Peasley. Also Birdo toward Popple, depending on what gender Birdo you think is.
There are some people who can't help quickly getting sick of Prince Peasley's constant hair flipping.
Seinfeld Is Unfunny: Now that the main Super Mario Bros. platformers make a point of having Luigi playable and prominent, the jokes about his second-fiddle status and him staying behind to "protect the homefront" can lose some of the impact they originally had in 2003.
Trunkle is a real pain. The worst part about the mutant Sudowoodo thing is that there's no indication that you have to (let alone the fact that it's even possible) attack the orange in his hair instead of the beast itself.
Whichever friend of Jojora that you pick, it'll end up as this trope. If nothing else for taking too long.
While Cackletta's Spirit isn't quite one, the beginning of the battle is. Both Mario and Luigi have only one hit point, and she gets to go first, unleashing a barrage of attacks that the player knows nothing about. And losing—that is, getting hit even once—means you have to fight Bowlettaall over again.
That One Level: Joke's End, in large part due to extended sequences where each Bro has to explore alone through enemy-filled territory and due to the labyrinthine design. Even the final dungeon is easier to navigate.