SSE was awful because it didn't take advantage of the actual physics of the multiplayer and was just a poor man's Kirby without copy abilities. It was boring to slog through while the Adventure Mode in Melee is short enough that despite how "barebones" and simple it is, you don't get bored of it -- mostly because all of the "stages" were based on the actual franchises and the physics didn't change between that mode and the multiplayer. Plus, they mixed it up between the normal combat on stages in the MP and platforming-like stages such as Mushroom Kingdom and Mute City that were short but ridiculously cool homages to the franchises (especially Mute City).
Everybody moves slower in SSE, everybody falls even slower in SSE -- I don't know why they thought it was a good idea. It made the platforming non-difficult, the only thing SSE had going for it at that point was the combat. And the combat was awful because the enemies were glorified Kirby enemy wannabes and reacted that way (except for the one or two levels where they actually had Koopa Troops and Goombas). It's fine for the bosses to, you know, not flinch but enemies barely flinching and being able to move the way they were and attack the way they were made combat more frustrating than it was fun.
Just my opinion, anyway. I'm glad SSE is gone, I play Smash for multiplayer. It's no secret SSE was the majority of Brawl's development and why it got delayed so many damn times. No SSE is the reason why this game won't be delayed to infinity and I'm glad for just that alone.
