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.
Fridge Brilliance: Nintendo nowadays is saying that Mario developed his abilities when chasing DK to save Pauline. Considering that this game actually birthed many mechanics the series would come to tout in later installments (such as the backflip and triple jump) it all comes together nicely.
Good Bad Bugs: The fact that closing gates are stopped by any tiles Mario can interact with (including temporary ladders) results in an Easy Level Trick for at least one stage. Stage 2-3, to be exact.
Goddamned Bats: In the most literal sense, they can't even kill you by themselves but they are very annoying.
The sequence that plays before the title screen is rather chilling: a hole has been broken into a brick wall and eyes peering out from the darkness of the hole with ominous music to match. Good luck trying to sleep after that; it will haunt your mind forever.
How about the final level? You beat stage 9-8, Donkey falls to his doom. What's this? Stage 9-9? Did the giant DK head just smile!!?
Stealth Prequel: Depending on your viewpoint, as this is a retelling of Mario's first adventure, and in the ending sequence, it appears that World 1-1 from the original Super Mario Bros. was on the other side of that tower. Not only that, but it seems Pauline introduces Mario to his first Super Mushroom.