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.
Anti-Climax Boss: The final Bowser is basically unchanged from every other one. Like the fake Bowsers in worlds 6 and 7, he tosses hammers, but that's it. The real challenge is just getting to him, much less with a power up on hand (in world 8, not likely).
Getting to Bowser with an extra hit turns him into a complete joke no matter when you fight him. Just take a hit and then run to the other side and grab the axe before your Mercy Invincibility wears off.
Awesome Music: The Overworld theme is probably the most well-known song in all of gaming. The Underwater theme isn't too bad either.
Demonic Spiders: Hammer Bros. and Lakitus. The former throws hammers at an absurd rate that makes it hard to dodge and attack them, especially since touching the hammers at all counts as a hit, and jump a lot making it hard to hit them. Their jumping can often make jumping on them a risk, because unless you hit the top of them, then you get hit instead. They can also jump downward from platforms, meaning if they're on a set of blocks, trying to hit them from below can easily result in them killing you. Lakitus... oh boy... those bastards are on clouds at the top of the screen, and it's extremely rare to come across a point that lets you get high enough to hit them. For attacks, they drop spinies, which you can't jump on. So to recap, you, for the most part, can't hit them, and they drop a constant supply of enemies you can't kill unless you have a fire flower.
Everyone Is Satan in Hell: Basically, as some players noted, Mario's design resembles Soviet leader Josef Stalin, Mario is the hero of the game, and the ending of a level basically depicts Mario putting down a flag that slightly resembles the peace sign and hoisting a flag that has the Soviet Red Star. In one Nintendo Power issue, this subject was discussed in the mailbag section, although the staff denied the resemblance, noting that Mario collects coins and is saving a monarch, even trying to put Mario's moustache on the Russian boxer/soldier from Rocky IV. It's much clearer in later games with more advanced graphics: the symbol of the flag that Mario pulls down is supposed to be a Bowser skull. In its 8-bit form, it resembles a human skull or a Koopa shell.
First Installment Wins: Sort of. While future games in the Mario series are just as, if not even moreso beloved in the fandom, this first one is undoubtedly the most famous.
It Was His Sled: The princess is in world 8. When this game was new, players might have wondered how many times Mario must go to another castle. Since then, several more Mario games have kept the princess in world 8 or chapter 8. So now players expect to find her in world 8.
Killer App: For Nintendo, for the NES, for the home console video game industry in general.
Polished Port: Super Mario Bros. Deluxe on Game Boy Color has a number of features that make it worth keeping: being able to save your progress and select any stage you've completed so far (even in it's Lost Levels port, which is necessary given how difficult it is) a Challenge mode for each level, and a 1 and 2-player race mode. Unfortunately, the screen resolution was significantly cropped to fit the handhelds small screen (and to keep from squishing the sprites down), which can make gameplay more frustrating than the original. The port does have some tweaks to make up for this, such as allowing you to reorient the screen to look above or keep the view locked ahead of Mario, in addition to tweaking the Ratchet Scrolling so you can slightly backtrack to an out of view area, but it only helps so much.
Seinfeld Is Unfunny: Even though the game was a massive success and people still look back on the game as one of the ways of making a game being done right, even long time fans agree that games like Super Mario Bros. 3 and Super Mario World did a better job in showcasing what Mario can do and how the games should be. While the original game is never considered to be bad, it can be barebones when compared to the later games in that era.