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.
Demonic Spiders: Heaven help you if Mario or Luigi decide they want to be in first place. With their infinite stars, you will never stop them.
Ensemble Darkhorse: Some say he is a good character, but it seems everyone either loves or hates Koopa.
Game Breaker: The Feather often allows racers to jump over walls for huge shortcuts in a game where traditional shortcuts are rare and minimal. Because of this, the TAS abuses it to no end.
The heavy characters (Bowser and Donkey Kong jr.), despite their low acceleration, have the highest top speeds in the game. For expert players, Bowser and DK are easily the best choices for tearing through courses at top speed— provided they don't hit anything. The inverse is true for the lighter characters— they have better acceleration, but their top speeds aren't as high as the heavyweights.
Goddamned Bats: plenty of obstacles, but particularly the Monty Moles that malevolently jump out of their holes just in time to hit your kart, stop your momentum entirely, and slow you down until you press R enough.
Nausea Fuel: Motion sickness from all the driving, moving backgrounds, crazy colors of Rainbow Road.