-- Sean Malstrom, Purpose of 3DS
So, Mario started off as THE 2D platformer, beginning life as a unique skill game with a designated ending. This game was a bridge between the endless challenges of arcade games and the ending contests of carnival games. Donkey Kong Jr. followed suit. Mario spawned a small line of arcade platformers: Wrecking Crew, Donkey Kong 3, and Mario Bros were all made available. Mario was quite recognizable as a 2D platform arcade character, but it was not until 1985 that he was made a global icon.
Super Mario Bros. was perfectly designed, and set Mario in place as a worldwide icon for video games. Everyone knew what Mario was and did. This game sold system after system after system. The Great Giana Sisters were an attempt to ape Mario, and became infamous for it. There were countless other clones. Platformers went on to flourish. Mario had to make a return trip. Japan's attempt was... not good.
It was basically a highly difficult mission-pack for the first game. There were new levels and some new objects and elements, sure, but it felt too close to the first to be anything special. We'd already seen similar things, but they were for specialized purposes: VS., Special, All Night Nippon... This was the "sequel" to SMB1. Japan flunked, and Nintendo of America knew it! Instead, Nintendo took Miyamoto's discarded plan for a new Mario game, which had already materialized as Doki Doki Panic, and decided to build a whole new Mario around it. And it worked!
The Mario TV show combined both Super Mario Bros. games equally for elements. Building a fantastic world was what Mario was about. Super Mario Bros. 3 learned from this. Mario was a flagpost, THE flagpost even!
This main Mario product vanished in 1998 after the failure of SMW2. Mario had no main focus anymore, which was VERY evident in the Gamecube era. The DS brought him back to his prime stomping ground, and he mostly kept there throughout the future. However, these games decayed over time. People noticed this decay, and as Nintendo tried to bridge the gap, they simply hoped 2D would get better, and felt they were forced into 3D. Too bad most 3D games felt the decay as well.
Hence for Mario, the world is flat. Yet I don't understand why Malstrom changed his tune to thinking 3d is inherently bad.