Dueling Games: With the first Knack game, which was released a week or two later, depending on territory. Knack got worse reviews than this game, but actually sold a fair bit better between its launch and around Christmas 2013. Though this game would ultimately outsell Knack quite handily, many agree in retrospect that this was the point when it became obvious that the Wii U was going to lose its console generation, badly. The Mario franchise would subsequently get payback in 2017, when Super Mario Odyssey handily demolished Knack II in sales.
People actually guessed a playable Rosalina after getting a look at a screenshot that showed outlines of all the possible stamps for use on Miiverse. That screenshot showed that there were five icons that resembled the player character icons used in the user interface (Player selection and life counter).
The Other Darrin: Rosalina is voiced by Laura Faye Smith rather than her previous voice actresses Mercedes Rose and Kerri Kane. According to MarioWiki, Kane and her family moved.
Throw It In!: The Double Cherry powerup was created by accident when a developer placed two Marios instead of one. The team loved the concept so much they decided to add it in.
What Could Have Been: Peach was not always intended to be playable. At some point, she would still be the Damsel in Distress, and the Sprixie Princesses wouldn't have existed if that decision was made. But then Yoshiaki Koizumi suggested to have her within the playable roster.