One of the most disappointing things about the reveal of Mario 3D World at E3 wasn't the plain graphics and music, but how barren, straightforward and simplistic the levels were. Gone were the inventive twists and clever puzzles the series is known for, replaced by levels that looked almost auto-generated.
At first (well, second, after I got done putting a hit out on Koichi Hayashida) I just thought they'd rushed it out to fill the Wii U's barren software schedule, and hadn't taken the time to handcraft each level well.
Later, however, I realized that the simplistic levels were actually a casualty of its new focus; multplayer in 3D. In order to keep all players in the screen and give each of them a good view of the environment, the worlds pretty much had to be one-sided and simplistic; if you had too may obstacles they'd obscure players and the features each one might need to see.
In their rush to try to "3D-ize" NSMBW/U's 4-player mayhem, Nintendo let it ruin the main draw of 3D Mario; its genius 3D Worlds. (Oh, the irony)