After epic adventures on NES, and the SNES, Mario need to jump straight into full 3D. Super Mario 64 changed the Mario gameplay to make it fit the 3D change instead of it being Super Mario Bros with 3D polygons instead of sprites.
Walking around the huge world was amazing at the time. It seem like there was so much to explore, so many secrets to find, and even random glitches to exploit like trying to jump on top of the castle without using the cannon.
The only things I didn't like about Super Mario 64 was the lack of variety in the bosses (most of the time it's the same Bowser fight), and not too many power-ups this time. The N64 original is king among platformers though, and I'm excited to hear that Super Mario 64 DS can use the 3DS's analog stick.