It's the sheer wasted creativity of it all. I recently replayed it, and there were some truly brilliant moments among the mediocrity, that may have even surpassed parts of the original games.
I'm talking about stuff like the Enigmansion, Gooper Blooper's rhythm attacks, the Bafflewood, the Snifit game show, all that stuff. It was brilliant ideas like that that made me want to keep playing this.
But why, even with all these great moments, was the rest of the game so mediocre? IS clearly showed in these parts that they COULD make an amazing game, but they didn't. --- "There's no point having the biggest and most explosive gun operated by someone who can only use a slingshot. That's what the Vita is"
What specifically, IYO, makes this game "so mediocre"? I'm not going to argue whether or not it is- but what makes you consider this game's brilliant moments "wasted potential"?
I would consider the level design and environments to be consistently excellent. The most common complaints that I've read have been regarding the battle system, the story, and the lack of in-game hints; These aspects of the game were also consistent through the entire game.
If youre looking for an answer for why this game has been widely considered mediocre (I wont assume these were your qualms), I think thats it. If you think the creativity put into this game was somehow wasted, I have to disagree. I dont find it hard to appreciate the beautiful environments that IS created despite the game's well-recognized flaws.