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.
Broken Base: The DS port. On one hand, it adds a ton of new content, like new tracks, new modes, new characters, new music, and even new fun doodads to mess with (like the Sound Recorder). On the other hand, the new quickstart is a Scrappy Mechanic, the hovercraft is much more difficult to control, Banjo and Conker are lost to the copyright gods, and trophy races and battle modes are no longer possible in single player. Fans are very divided.
Tiny Kong's older Ms. Fanservice redesign in the same port. Some loved it, while others missed old Tiny and thought fanservicing up a child character was creepy.
Hot Top Volcano, one of the most iconic tracks from the game (in both the musical and gameplay senses.) It's timed exactly such that when you typically hit the final lap and speed up, the drum breakdown kicks in...
Strangled Shrine, used for one of the DS-exclusive tracks. It's evocative of the already fantastic Hot Top Volcano theme, but more watery, which is pretty much what Strangled Shrine is. The brass stings are pretty epic, too.
Fridge Logic: Why does Taj the helpful genie have all the balloons you need to win the game (and withhold them from you until you win a race)? Wizpig stole them all.
Fridge Brilliance: Perhaps when he first appears, Taj had just finished taking the balloons back, but he doesn't consider himself ready to take on Wizpig and decided to give the balloons to the racers as they proved themselves ready over the course of the game?
Game Breaker: If you unlock T.T., practically every race becomes a joke.
The epic whooshing sound that you hear if you activate a level 3 speed boost.
T.T. saying "Power up!"
Nightmare Fuel: This cutscene that plays when you go back out into the main hub area after collecting all the pieces of the Wizpig Amulet.
Obvious Beta: A very rare (no pun intended) example of this not having any real negative effect on the game's enjoyability. The game was rushed to market so that Rare could have something out for Christmas 1997, and numerous signs of this are left in the game, including the fact that attaining 100% Completion requires two save slots (one for the normal adventure and one for "Adventure 2") and several not-quite-functional elements accessible via in-game cheats, including a 2-player Adventure mode and a rather-broken sound test (all music tracks are only listed as numbers, are in no real order, include some beta elements, and exclude some music heard in the final game, and the Variable Mix tracks do not have their individual versions listenable - only a mash-up of all versions at once). The core game itself is complete, however - many of the Obvious Beta elements are side-features that aren't in any way necessary to enjoy the game itself.
This could also explain why Bubbler the Octopus is the only boss not seen in the ending.
Padding: One gets this feeling given that the single player mode requires you to, for each world, race on all the tracks, beat the boss, collect silver coins in each track, beat the boss again, and then face Wizpig when this has been done with all worlds. If you want the real ending, you need to enter a grand prix to win a trophy in each world, beat the bosses again, and rinse and repeat the initial and coin races in a new world before facing Wizpig one more time.
Stop Helping Me!: It is rather easy to trigger dialogue with T.T. as he's wandering about the level hubs, even if you don't intend to. Especially pesky in the DS version, where he doesn't actually do anything useful besides make comments.
That One Boss: Bubbler the octopus. And the second race against Wizpig himself.
Wizpig is somewhat excusable, given his role as the Final Boss. The second race against Tricky... argh.
Tricky's not too tough if you know where the shortcut in the mountain is.
Bubbler's second race is considered by many people to be the toughest race in the game, next to Wizpig's races on Adventure 2.
The first race against Wizpig is damn near IMPOSSIBLE unless you know what to do. You need to use the zipper trick (Let go of the gas before getting a boost) to beat it. Unfortunately it's quite possible for players to never to get this bit of info, since it only shows up randomly after bosses. If players don't know the zipper trick, the one way to beat Wizpig is trying to get a boost start and stay in front of Wizpig all the time, hoping his foot will hit the player as he runs, pushing the kart forward. It requires insane timing as messing the position of your car up will result in Wizpig squashing you. Wizpig's first race is either a Guide Dang It or a Luck-Based Mission.
That One Level: Many of the normal levels during the Silver Coin Challenge, but especially Greenwood Village.
This gets much worse in Future Fun Land, particularly Spaceport Alpha.
That One Sidequest: Unlocking T.T. as a playable character requires beating his best time on every track. And he's good. In the original, this means unlocking the fifth and final world... In the remake, this means both unlocking the final world and buying all the tracks in the D.K. Jungle world. Of course, once you get T.T., you have no excuse for losing to anyone ever.
In the remake, getting all the balloons in the "Balloon Touch Challenge" can be frustrating for some courses. In fact, it is oftentimes the courses that are otherwise the easiest to race on where they are the hardest, since those courses are shorter and so there are more balloons in one area, forcing you to tap like mad to get them all before they pass by for good.