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. 3
Anti-Climax Boss: Your final fight with Bowser. You can exploit the fact that you can touch Bowser's lower part of the body and not take damage. (If you're in Super form, just crouch.) Add to the fact that the floor crumbles every time bowser stomps on you, and you just have to wait until when you have to dodge on the very moment Bowser slams through the final layer of the floor and falls to his doom. This is Boss Arena Idiocy at its best.
Although not quite as awesome as 5-3, 4-6 contains doors that change the size of the enemies between the giant size typical of world 4 and the standard sizes found elsewhere — kind of a 2D precursor to Tiny Huge Island from Super Mario 64.
The treasure ship, which shows up if your score and coins meet a certain criterion. When you go to it, it's kind of like a Doom Ship, except covered in hundreds of coins. It is glorious.
Breather Level: Level 6-8, aside from the fortress levels, has absolutely no ice gimmicks at all and it is the only level whose design looks like it belongs in World 1 being made up of grass. The level itself is straightforward and contain a lot of chuckable blocks.
Actually, most worlds have at least one level that has little (if anything) to do with the main theme of the world (and not just the castles and doom ships).
Demonic Spiders: Boss Bass, who can be found hanging around most of the stages in World 3, and has an annoying habit of swallowing Mario whole irrespective of what power-up you're using, instantly costing you a life.
However, the TRUE ensemble darkhorse is Kuribo's Shoe. Despite appearing in only one stage, of one world, of one game, it is enormously popular, so much so that some fan-made hacks, such as Mario Adventure, made sure to let it carry over to every level.
Several levels in worlds 6 and 7 require ducking through a one-block-high passage to continue, which cannot be done if the player is in the Frog Suit. On the other hand, why a player would be using the Frog Suit in levels that don't have water…
It's impossible to do without cheats, but using the Kuribo's Shoe in a water level will cause the player to sink to the bottom instantly, rendering some levels completely impassible.
Woolseyism: The Koopalings weren't actually named in the Japanese version of the game. Their individual names were added in by the English localization, which explains why the DIC cartoon adaptation gave the Koopalings vastly different names.