The platforming follows a tightly-set and excellently integrated mechanic of 2.5D, being one of the first games to properly do this, in fact, even if it never gets the credit for it.
I also really love that it's a game that allows perfect integration of your enemies. You can throw them in all directions, including down to unleash a double jump. They're actually your primary means of attack; you can't attack enemies in any other way than grabbing them/throwing other enemies at them. They're an absolute necessity to playing the game properly, and many parts of the game could NOT be solved without the very much necessary enemies around to get you through it. As well, enemies are very unique and get different and even more difficult as you advance through Phantomile. Some can't even be grabbed, and some have to be killed more strategically.
And I ADORE the characters and the story. Both of these work excellently to make an emersive and fun world to explore and enjoy. The characters, though normally minor outside of Klonoa, Huepow, and arguably Karal, are all very unique with different mannerisms and are a huge blast to interact with and help out along your adventure.
The story itself is simple, but not without its twist and turns. It's fairly light-hearted, but when it reaches a turn... well, no spoilers, but it'll catch you off-guard.
Plus, on a mostly-related note, the cutscenes are SUPERB. For a PS1 game, the cutscenes look absolutely beautiful and downright breathtaking at times. They're few in number in terms of cutscenes outside of sprites, but they look AMAZING. Better than the Wiimake, that's for sure.
And if none of that is enough, it is the CUTEST THING EVER CREATED. The characters speak an adorable mix of gibberish and fully-developed fictional language inspired by Japanese, known as Phantomilean. Klonoa himself is the most adorable thing you will ever see, being a small little child of a cabbit, with extremely long ears, a dog collar, big yellow eyes, red shorts, lack of a shirt, and he's just... D'aww...
I love the game to death, and I love EVERYTHING it has to offer. It's more than worth the six bucks one can buy it for on the PSStore (since finding the original is expensive as all hell.)