"Link Kart 8" or "Morton Kart 8", for how popular those characters are among players (the latter due to the double-whammy of being a heavyweight, statistically the best choice for a high top speed, and a Koopaling; and the former simply because of the Zelda series's popularity).
In Spain the game is popularly called "Kartocho", a portmanteau of "Kart" and "ocho" ("eight" in Spanish).
Killer App: Considering how well the original game sold — approximately two-thirds of all Wii U owners purchased it — it very likely prevented the system from going down as one of the biggest failures in videogaming history instead of "only" being Nintendo's lowest-selling home console. Weirdly enough, Mario Kart 8 Deluxe also ended up being this for the Nintendo Switch to some extent; while not the killer app for the system (that was The Legend of Zelda: Breath of the Wild), it was still held up as one of the biggest reasons to purchase a Switch, and ended up outselling the original version in just under a year. By the end of 2018, it had dethroned Super Mario Odyssey as the top-selling Switch title.
Limited Special Collector's Ultimate Edition: A deluxe package which came with a figurine of the infamous Spiny Shell. It was a Nintendo Store exclusive in North America but was more widely available in the rest of the English-speaking world, who also got some other little trinkets depending on where the package was ordered from such as key rings or a Bullet Bill shirt.
Older Than They Think: Using a Pac-Man amiibo in game unlocks a costume based on the character. This isn't the first time Pac-Man appeared in a Mario Kart game though — the Namco-developed and arcade-exclusive Mario Kart Arcade GP games feature the yellow circle and several other Namco characters.
Surprisingly, this isn't the first kart racer to feature anti-gravity: Crash Nitro Kart did the same thing over a decade prior (though, admittedly, Mario Kart 8 uses it in much more interesting ways).
Urban Legend of Zelda: There are many claims online that honking the horn just as the race starts will cause the computer players to blow their engines and have a delayed start. This is entirely coincidental; honking the horn is just a funnyEaster Egg.
What Could Have Been: When thinking up ideas for what new gameplay concepts to introduce in this game, the development team initially considered being able to drill through the ground.
In the second DLC pack, 3DS Shy Guy Bazaar was originally supposed to appear as a DLC track, but was replaced with either GBA Ribbon Road or GBA Cheese Land, as shown by its code name in the files.
This is the first Mario Kart game where no tracks (new, old or DLC) share the same music (although most parts of Mario Kart Stadium and Mario Circuit themes share the same tune, but with different instrumentation). This is broken in Deluxe, though not entirely: the music for the battle stages of Sweet Sweet Kingdom and Dragon Palace are slightly remixed variations of the music respectively for Sweet Sweet Canyon and Dragon Driftway.