No one was expecting the Super Smash Bros. solo experience to be amazing. And it's not. It has a few flaws, particularly with co-op mode.
Unfortunately, the Smash Bros team decided that the SubSpace Emissary had to be better, or at least longer. So they commit one of the worst sins you can: they make the player go back through the game again. You have to defeat every boss, and run through every level, all over again. Not entertaining. This is particularly bad because beating the SSE is the easiest way to unlock several characters.
I'm not saying it's hard; it's not. It is tedious and repetitive. It's the definition of repetitive. My question: Why? No one was expecting an amazing campaign mode. The Great Maze adds nothing but time. There are no cutscenes during the Great maze, and no new materials. It is clearly an attempt to lengthen the game.
It hardly kills the game, but it's a mystery.