Let me start by saying that I've had the game since Thursday and that I've completed 100% of the SSE in 10 hours and 6 minutes (and that's only because I couldn't find 1 trophy box in one level so I re-played it at least 6 or 7 times).
It's just dull. Really, really dull. The levels are overly simplistic and boring. The flaw is that they're designed so that every character can complete them which makes sense since there are a variety of different characters. At the same time, the entire mode fails at being fun and/or interesting. Nothing about it is especially original (i.e. choose-the-correct-door-to-proceed-puzzles, hit the switch to open a door, find the key which is is often right next to the locked door). They even recycled parts of the older levels to make the last level. That's just plain laziness. Additionally, none of the levels really take advantage of specific character attributes. For example, characters with extended jumps (i.e. Kirby, Peach etc.) never need to use them to advance. I found plowing through levels with Ike to be the easiest way to get through the mode which I completed just to get all the characters.
The enemies too are poorly designed which I don't understand. Game Arts has excellent artists (i.e. Lunar, Grandia). More importantly, they're all pretty easy to defeat even on higher difficulties. Some of the bosses offer some challenge, mainly the final boss (who has 1 super cheap instant kill move and a couple other super cheap almost instant kill moves) and Meta-Ridley.
The story... well, there isn't really a story. The characters are all kind of just there and nothing really makes sense or gets explained. Some sh*t goes down and they all just decide to team-up. But, there is a ton of CG which gives the illusion that there is a coherent plot behind it all because why would they have put all those cut-scenes in there otherwise?
Seriously though, my problem with all the CG is that it hogs disc space which is the reason the game is dual-layered which, then, is the reason why load times are so frequent. Maybe I'm spoiled, but when Melee had minimal load times, I'm having a hard time understanding why Brawl, developed some 6 years later, has so many.
I know I'm ranting, so I'll end this soon. I remember months and months ago when Smash_Brother wondered why the development team spent so much time on the adventure mode when Smash Bros. is primarily a multiplayer game. While I always wanted an expanded adventure mode, I have to agree with Smash_Brother which proves that there's a first for everything. The idea was better on paper or maybe it was just poorly executed here. Either way, think of all the characters and stages we could've gotten instead.
By the way, the game is still near perfect because Smash Bros. is inherently a multiplayer game and they did get that part right though maybe its a little too slow (maybe).