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.
Breather Level: Rainbow Road, despite being a ridiculously long course and having Chain Chomps, is far easier than the previous level, Banshee Boardwalk.
Character Tiers: Almost nobody likes the Heavy characters, they are incredibly slow and useless on turns and offroads. This game probably introduced a real tier system.
That said, there are skilled players who know how to drift boost (drifting is how you don't lose speed while turning after all) and use the characters' weight to their advantage. And really, if you're going off-road without stars or mushrooms you're not playing well anyway.
Ensemble Darkhorse: Believe it or not, a Thwomp got this treatment- the green Thwomp locked in Bowser Castle, actually a regular Thwomp with a lighting in the cell that makes him appear green. Fans gave him the Affectionate Nickname "Marty" and posted hackneyed ideas about how to unlock him.
Game Breaker: If you happen to have a Spiny Shell and hold Z, you'll trail the shell behind you. As long as you don't release Z and shoot it, it will stay behind you, providing yourself a pretty wicked shield. Since the shell only destroys itself when hitting the racer in first and has priority over all other items, it will not go away no matter how many items (or racers) hit it. Just don't shoot it once you're in first place.
Yoshi Valley is a huge labyrinth on narrow cliff sides, and the computer doesn't keep track of your places. Oh, and there is a huge egg near the end that can crush you.
Banshee Boardwalk is easily the hardest course in the game, lots of very sharp turns and drop offs, with very little room for error.
Toad's Turnpike on Extra Cup, where not only the turns, but the cars change direction.
Wario's Stadium has a huge jump in the middle of it which if you manage to fall in the middle by screwing up or getting hit by something while jumping, Lakitu doesn't carry you back to the other side and instead you're forced to race back into the ramp to do another jump. It'll screw you up so badly in the rankings that you've pretty much automatically lost the race.