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.
Awesome Music: Any of the remixes that were put into the game, and Galaxy's Floater Land Theme.
Game Breaker: Unfortunately for the designer, there is obviously no way to ensure an entirely linear way of game progression, since certain areas can actually be accessed prior to their storyline appearance.
Hover FLUDD. Lose your balance on a platform? No problem!
Most Annoying Sound: Without an awesome sound card, the random bursts of static. KRRSH! KRRRRRRRRRRRRRRSH!
Playing some of the later levels (such as Lethal Lava Land and Tick Tock Clock) can be a big pain in the butt without Hover FLUDD.
Also, the Collapsing Lair segment of Bowser's Castle after Bowser is defeated. The area's covered with lava, and FLUDD is not able to be used whatsoever.
They Just Didn't Care: Luigi was obviously a rushed character. Although he has some new voice clips, many (such as falling from a height or collecting a Shine Sprite) are Mario's voice clips reused instead of using the Super Mario 64 DS equivalents. Also, he "turns" into Mario whenever he drowns or throws Bowser during the final boss battle, as well as during cutscenes.
Additionally, proofreading the text could have been nice.