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.
Author's Saving Throw: Mario Kart Wii got released in Europe first. This is most likely in order to make up for Nintendo's earlier announcement they would release Super Smash Bros. Brawl in June 27, 2008, that is, three and a half months after North America's March 9, 2008.
Base Breaker: Waluigi. His exclusion from Mario Kart 7 has really divided the fans over whether or not that was a good idea. Fortunately, he returns in Mario Kart 8, to the joy of many fans.
Maka Wuhu was this for the online players due to the infamous cliff-falling glitch that puts the user of it ahead of others by several seconds. One portion considered it unfair for those who don't know about it or just want a simple race while the other portion said it's fair game for anyone since it's in there. Nintendo's response to the outcry by keeping said glitch did not help ease the fire, so to speak.
Best Level Ever: 7 introduces courses that forgo the usual roundabout design, instead using checkpoints rather than one finish line. Most people regard these as the best tracks in the game, especially Rainbow Road.
Waluigi's Pinball is regarded as one of the better courses.
Breather Level: Mario Kart 64's Rainbow Road, unlike the tracks before it and its Super Mario Kart counterpart, is merely extremely long, and, unlike later incarnations, has rails on the entire track instead of just the curves.
Character Tiers: In every single game, but most notably in the 3-D games. Mario Kart Wii had each character give invisible bonuses, with the more favorable quickly reaching Tier-Induced Scrappy levels. 7 seems to have averted this with the customization system and lack of significant character bonuses, however.
Complacent Gaming Syndrome: Almost a standard in the Wii version where most people will only use a bike and certain characters just for the sake of being slightly faster than others.
May be a factor in why they were removed in 7.
Speaking of 7... Hope you like racing against Metal Mario!
You can probably find Lakitu somewhere as well.
Frequently finding Metal Mario among competitors can be explained by the fact that Metal Mario, while a Heavy-class character, is one size smaller than the usual Heavyweight characters, and thus, a smaller hitbox.
Fan Nickname: The infamous Spiny Shell is pretty much known to all its fans and haters alike as the Blue Shell. That, or a long string of profanity.
Game Breaker: Snaking since Mario Kart Double Dash. Removed in Wii.
Base Breaker too. You're either a cheater who can only win when they do it, or a noob who only hates it because they can't do it.
Bikes in Wii, they massively overshadowed karts to the point where some players joked about the games title being 'Mario Bike Wii' (or 'Funky Bike Wii' due to Funky Kong's similar status as a character version of the trope.)
Or Daisy Bike Wii if people want to use someone slightly different.
The series have tracks that can cut down on your laps greatly due to abusing exploits of the checkpoint system (this determines where Lakitu drops you off if you fall from the track). A notable one is the glitch in the Grumble Volcano track in Mario Kart Wii; if done correctly, the player can literally finish the race in 30 seconds or less. Naturally, nearly everyone abused the exploit online and it was more obvious when people started to spam vote for the track. Mario Kart 7 also has a similar one with the Maka Wuhu track that allows people to skip huge sections of the track. Not only people began to abuse this online as always, but the exploit was found almost days after the game was released.
Several items are game breakers depending on the game they are used in:
Super Mario Kart had the mushroom give a huge boost and the star gave players insane speed boosts for several seconds which allowed them to cut across tracks with dirt/water/sand, etc. These items were nerfed and adjusted to be less game breaking but still useful.
The multi-banana peel in Mario Kart 64 gave you a trail of 5 peels, which meant up to 5 protections from shell attacks and also 5 peels to spray the tracks with. Later games reduced the item to 3 peels.
Mario Kart Super Circuit gave red shells a secondary function by allowing players to drop it behind them and the next player that passes it gets attacked by it. This made it too easy to place it by gaps or jumps in order to make people fall off the track, so the function got removed in future games.
Yoshi and Birdo's egg attack in Mario Kart Double Dash!! released several items when it broke and one of them could be a star.
Nearly every special item in the game is a game breaker; the fireballs are a spreadshot that can hit multiple people, the giant shell is basically a faster and bigger version of the green shell that's difficult to avoid, the giant banana peel is also difficult to avoid and it splits into 3 smaller peels when struck, and the hearts allows you to absorb up to two items, including most special items, for your own use later.
The Boo item turned the player invisible, which made them immune to all forms of attack, and it stole another player's item. Players could also drive off road in the DS version without loss of speed with the Boo item as well.
Goddamned Bats: Some levels have hazards which can pop out at bad times to slow you down, including sliding walls, falling presses, and, yes, bats.
Sometimes, the goddamned bats can be the other racers themselves as they can get in your way when you are trying to pass them or crash into them while performing a power slide and losing your momentum. This is much more noticeable with the AI since they don't deviate from their preferred path too much.
Hell Is That Noise: You will panic every time you hear the alert of a homing item. Especially the Spiny Shell.
Most Annoying Sound: In Mario Kart Wii, characters scream things like "YEAH!" or "WOOHOOOOOO!" at the top of their lungs for almost everything they do, even for little things like mini-turbos and wheelies. Although certain characters get more attention for it than others, when it comes down to it, every character could be considered equally annoying due to the cartoony ear-piercing voices.
Arguably even worse in 7.
When playing with a Mii, get ready for "YEP YO! YEP YO! YO! YEP YEP! YEP YO!"
The Scrappy: The four babies in Mario Kart Wii for being insignificant to the point that makes Waluigi look like the main character. The biggest scrappy award goes to Baby Daisy, considering they made her up just to be one of the babies.
Honey Queen in 7, as she is a extremely minor character who had just appeared for a small handful of missions in SMG and SMG2 before and is widely considered as being undeserving of a spot in the roster, getting in over beloved veterans such as Waluigi or Dry Bones. Her horrendous voice acting doesn't help matters, either.
The Spiny Shell, if the examples floating around didn't make it clear enough.
This one's less of a bother in the DS and 3DS games, since you can see what items your opponents get, allowing you to take measures such as dropping back and letting someone else get hit, or just driving off the course out of pure spite. That said, it's still very annoying.
The Thunder Cloud from Wii is possibly even more reviled. The idea is that you get a big increase to your speed as long as you have the cloud above you, but it zaps you after a few seconds. The result is that the player has to choose whether to pass it on to an opponent and lose the speed buff, or keep it and risk getting shrunken. All in all, not too bad, yet for some reason the game designers made it so that it triggers as soon as you get the powerup, regardless of where you are on the track.
The Lightning Bolt as well, which it really shouldn't be considering it affects everyone ahead of the player with it. (The farther you are the longer it takes to get better but still) The problem comes from it literally being rigged to happen too often and making you often lose your defensive item just as a red shell is coming.
Coins in Super Mario Kart and Mario Kart: Super Circuit. You need to hold coins to increase your speed (the boost caps at 10 coins) and getting hit by items, crashing into course hazards, or falling off the track made you lose coins. Even bumping into another racer made you lose a coin! The games also made the coins an item to give you 2 coins instantly, but it could also screw you over if someone behind you has a red shell or the like.
Mario Kart 7 changes the coins mechanic slightly where not having a lot of coins won't totally screw you over and bumping into people won't make you lose coins, but you can't hold more than 10 coins and you will lose 3 coins (or two if your coin count is low) every time you get attacked or fall off the track, which makes holding onto your coins a pain in the ass. Luckily, coins regenerate. Unluckily, you need to collect coins to unlock parts for your kart and you will be playing for a long time since you will need to collect several thousand coins and even beyond 10,000 coins just to get everything. This is made worse due to only being able to hold up to 10 coins per race.
Toad's Turnpike. Extra Cup. Now, Extra Cup normally just mirrors the tracks. For this track, the course is mirrored...and the traffic cars are oncoming. Welcome to Hell.
Rainbow Road ends up being this for a majority of every game except its really long and safe version in MK64 (which averted the difficulty after the first Mario Kart), thanks to the general lack of railings and the abundance of sharp turns and narrow pathways. That's disregarding everyone trying to throw explosive flying shells at you.
Mushroom Gorge in the Wii version. At least 1/4th of the track is over an abyss with only bouncy mushrooms to launch yourself across to another mushroom until you hit the other side. Hitting the mushrooms at a wrong angle will most likely throw you into the pit. Using a Bullet Bill while over the mushroom pit can also be disastrous if the power up runs out when you're over the pit instead of a mushroom or the ground. Even simply going too fast with a mushroom item or a star can throw you into the abyss. To make matters worse, you also have to hope people don't try to target you with a red shell, spiny shell, or a thunderbolt as you cross the gap.
Of course, you also have to watch out for players who will strike opponents deliberately when trying to cross the gaps. Those who know how to play very aggressively with attacking items sometimes pick Mushroom Gorge specifically because it's so easy to set everyone else back substantially in this way.
Maka Wuhu in 7 Online, due to the infamous glitch. This has been fixed, as of mid May.