"Every level is the same" might have been wrong choice of wording, but I'll be damned if after two hours it didn't seem like it.
The gimmicks in Brawl are there, but they either don't change much about the gameplay or the levels they occupy overstay their welcome -- usually the latter, since on average a level featuring a certain gimmick or two lasts for ten to fifteen minutes. Also, thanks to the great maze, they have to be played through twice in one save file. (The first time, you don't even get to choose your character. That's fun, right? 35 possible ways to play a single level, let's restrict the player to two.)
You don't like the maze level in Melee? That's fair, although the very most it will last you is five minutes -- easily less since you can hop over the Link battle triggers by jumping over the center of the arena. The Metroid escape actually is interesting, because you're timed and have to effectively take advantage of your character's mobility -- also, it lasts 20-30 seconds, and you don't have to redo it if you fail the timer. F-Zero's race track is kind of bland admittedly, but it's so short and painless that I don't even notice I went through it. Everything else actually is enjoyable, the Kirby rush, Pokemon Stadium's pokeball frenzy, Corneria's "THERE ARE LASERS EVERYWHERE" phase, etc... All simple but effective gameplay gimmicks. I don't think that's padded at all; compared to SSE's method of extending playtime, it's far more streamlined.
Also, I don't get what you mean by having to play through Adventure with "every character individually". You only have to do that if you want to get all of the trophies, and it's not the only way to unlock any of the characters IIRC. Also, couldn't you say the exact same for Classic mode and All-Star? What about Boss Battles and Break the Targets?
By the way, I don't mean to say that the themes Brawl used are bad. But that's literally all I remember about Brawl's levels. Except for the few with Mario enemies, those were cool. The ice climber one, actually not as interesting as Melee's come to think of it....
Actually, one level that was fun in SSE was the short "Five Warriors" one. And that's because it was just a mosh pit of enemies, and you didn't have to deal with any sluggish platforming segments. That level took under four minutes to complete.