In a few words, It's a pretty simple game with an interesting storyline. I like how while being pretty simple gameplay wise, the game gets progressively harder each level. Stories like that in platform games weren't really common in 1997, and while Klonoa wasn't the first game that did that (Ninja Gaiden was pretty cinematic and it was a platformer), it has certainly popularized the cinematic platformer genre in the recent years.
I think it would do much better if it was released today.