Wired has an interview with Takashi Tezuka. Some tidbits:
- The team was purposely not given a deadline in order to make sure that no idea would be cut for time
- Multiple badges were considered but dropped. “It felt more like maybe something that you would see in a Zelda title,” says Tezuka. “It didn’t really feel very Mario-esque … Mario gameplay is a lot more straightforward and simple.” So for those questioning the move, that's the official reason (and one I personally agree with).
- One high priority of the team was to fill the levels with secrets. I hope this is legit and not just fluff.
- Every main course apparently has a different Wonder effect. The team supposedly collected over 1000 different ideas and then narrowed them down