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.
Awesome Music: The series in general has some pretty awesome tunes, but one of the composer's favorites seems to be first ghost house theme in the first game, which seems to have ended up being the series' standard ghost house music (March of the Minis in particular has multiple variations on that particular track instead of having more than one unique track like the original). The versions from March of the Minis return in Minis March Again! and are joined by a sped-up version of the standard version, with these being used for Plus Mode's version of Magnet Mania in that game.
The Mini-Mario theme is very catchy and has been in every Mario vs Donkey Kong game. In Mini-Land Mayhem, each level select screen got a variation of the theme and the Plus Mode versions were sped up.
Breather Level: The first two levels of Super Skywheel (world 8) of Mini-Land Mayhem are surprisingly easy for so late in the game. Other than the shy guy blocks, 8-1 looks like it could be part of world 1 and 8-2 would be right at home in world 3.
Easy Levels, Hard Bosses: In Mini-Land Mayhem, the Donkey Kong levels can be this, especially in world 5, world 8, and the final battle. ESPECIALLY in Plus Mode. This becomes inverted in Cosmic Adventure where the DK battle is one of the easiest in the game (and perhaps the most fun).
Hope Spot: In Mini-Land Mayhem, the triumphant music that plays after beating the first Donkey Kong battle is this, until he just gets up and takes Pauline to the second stage. You get a big one at the end of the first Final Ferris Wheel with the credits rolling and everything, then you have to play a harder version of the original game, literally a New Game+.
Sudden Difficulty Spike: Teatime Twirl (world 3) of Mini-Land Mayhem is this, from this point onward the levels are bigger, puzzles more complex, and the DK battle can be legitimately difficult if you're not paying attention. This is where it shifts gears from a platforming game with puzzle elements to a puzzle game with platforming elements.
Getting a high score in 4-3 is the most infuriating thing about the first Mario vs Donkey Kong.Nothing comes close to the level of bullshit that is this level and whoever designed it is a sadist. Given the way the first half of the stage functions, you have to have almost perfect timing at certain sections, especially in the second half, if you want to just barely beat the default high score by just one or, if you're lucky/good enough, two hundred points (Every second left on the clock is multiplied by one hundred and added to your score). Experience has shown that it is still possible to get the Star (which is needed to help unlock the Expert levels) by matching the default high score. Just don't expect the game to acknowledge it by highlighting your score, because you didn't actually beat the old score.