This is less about Balloon Kid as a whole- a neat little platformer by Nintendo and Pax Softnica that serves as Balloon Fight's sequel, turned into an auto-scroller- and more about one specific thing that's always kinda fascinated me about it. For the most part, the game is pretty cute, with tiny enemies like birds and insects that can't even hurt Alice, your player character (beyond popping one of her balloons). This is how about 80% of the game is, as you can see in the screenshots above (the first one with the pencil building in the background? That's Alice's home town, Pencilvania, arf arf!)
But what really interests me about the game are the tone and feel of its later stages, primarily Stage 4 (the belly of a whale), Stage 7 (a giant cave) and especially Stage 8 (some kind of factory. These stages use slower, more foreboding music, and aside from some cute enemies like the octopi who can't hurt Alice, they're full of far more threatening traps and enemies like giant fang-like spikes that drop from the ceiling, jumping flames (admittedly seen in earlier stages), faulty lightbulbs that can explode and launch deadly sparks, and flame-pillars that spit out some of the strangest sounds you'll ever hear from a Game Boy.
It's just a little unusual for a game that goes from being pretty cute and relaxed- listen to the first stage music, how laid-back is that?- to developing a mean streak like that, even if it's only mild. It's accentuated by the Japan-only Famicom retooling, Hello Kitty World, which alters the enemies and graphics in these areas to be far less intimidating, taking away this odd little mean streak and, with it, some of the charm of the game.
Anyway, you can get Balloon Kid on the 3DS eShop if you've ever been curious, it does take a little time to really get going, but it offers a decent challenge by the end, so give it a try!