I don't like how the game is organized either, but it does the same sort of thing every modern action game does. It breaks up the game by requiring multiple plays to unlock everything, and has multiple difficulties which I feel compromises the balance of the game-- since it's up to the player to find the right difficulty, which is sort of a chore. I'm probably in the 0.0000001% minority here who would love for there to be just one difficulty mode (a really tough one, that would take weeks to beat something just a few hours long), one campaign, with all the characters selectable from the start in single player. I definitely like the game, but probably not enough to play through it more than once in single player, much less on multiple difficulty modes to try to find the "best" experience.
The way the stages are designed as hub worlds with missions made me think this would work great in a Smash Bros. game-- and it fits the style of the game since it's more of an arena brawler than a traditional beat-em-up-- but I think I was secretly hoping this game would be more like Spikeout than Power Stone. I guess it's sort of a mix of both.