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.
Lemmy Koopa is a lot easier than Wendy O. Koopa, the only real difficulty coming from the fact that his boss room has an uneven floor, which makes it tricky to use fireballs against him.
Iggy Koopa is also easier than the bosses either side of him (Roy and Ludwig), especially since his pattern is a lot easier to nail down than those of most of the other bosses.
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.
The Koopalings went on to become an entire Darkhorse Ensemble.
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.
Even Better Sequel: As important as the first game was, many regard this as the game which really set the bar for how platforming games should be done, and still consider it one of the best games ever made.
That One Boss: Ludwig von Koopa may be an even harder boss than Bowser. Every time he jumps he causes the room to shake for several seconds, immobilizing the player character. Worse still, the room is laid out in such a way that he'll often take a series of short hops from one side to the other, meaning there's no way whatsoever of avoiding him if you get caught in one of the tremors.
That One Level: The airship in World 8. It is not like the Koopalings' airships. This one scrolls faster, and consists mostly of small platforms with lots of space between them.
The tank level just before Bowser's Castle, which is loaded with cannon ball and Bob-omb launchers.
The latter third of World 7 will usually be enough to drive all but the most dedicated players to use their Warp Whistle to go to World 8.
World 7-4. Not only is it an underwater level, the screen scrolls automatically, and there is a minefield of Jelectros you have to maneuver through.
Several levels in worlds 6 and 7 of Super Mario Bros. 3 require ducking through a one-block-high hole to pass, something that can't be done if you're in the Frog suit. And in at least one of them, you can't even commit suicide by way of Bottomless Pit. Considering how difficult it is to maneuver on land with the Frog suit, why would you use it prior to entering a level with no water? If there are no enemies nearby to run in to, you can wait for the timer to run out.
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. If you're using an emulator which can disable cheat codes it's not so bad, but if you're using an actual NES with a Game Genie, you're screwed unless you can somehow bypass the water stages.
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.