Adored by the Network: In the Iwata Asks interview, Taro Kudo came out and said he hoped Sticker Star would be the new standard. Later, future products featuring the Paper Mario seriesnote Super Smash Bros for 3DS & Wii U and Mario & Luigi: Paper Jam indeed primarily/solely used Sticker Star as material, despite negative feedback.
Executive Meddling: The game doesn't feature any original characters besides Kersti because Miyamoto asked the team to create a game using only characters already made from the Mario world.
He also requested that the game shouldn't feature a story, hence why this game is lighter on plot.
Urban Legend of Zelda: In World 5-1: Shy Guy Jungle, one of the things Mario can find in the pile of trash is a serial key: "XD3R-B8HH-9ZR2-FL16". Some say that this is an actual serial key for something, but what it's for is unknown. It's not a Club Nintendo or eShop code, because both display the "wrong code" message instead of the "code has already been entered" message.
What Could Have Been: At one point in time, as early trailers and screenshots showed, partners were set to make a prominent appearance (including new partners that had never been seen in the series, such as a Toad and a Chain Chomp), and combat was going to be more like previous installments. However, partners were removed and combat was overhauled to better accomodate the sticker mechanics.
Unique mini-games were planned for Bowser's Castle, but were cut due to time constraints during development.