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.
YMMV: Mario Kart 64
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.
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: Great acceleration, speed, handling, and power slide in turns.
Mighty Glacier: These are the heavyweight racers have lower speed than the others, but lose less speed on turns, and can make other racers spin out on collision, and as a bonus cannot spin out themselves.
Bowser: Good speed and turning, has great strength (but not on par with DK) but is hampered by his poor acceleration.
Donkey Kong: The strongest racer in the game, who can effortlessly spin out smaller racers. Has great speed, but poor acceleration.
Wario: He has the best acceleration and handling of the heavyweights, but suffers from poor speed and is the weakest of the three heavyweights.