The new Super Mario for 3DS is an interesting new take on 3D Mario gameplay ... in that it largely restricts the level design to 2D. Or, at least, 2.5D. There are plenty of sections where you move in and out of the screen, sure, but for the most part, you're following corridor-style paths from left to right. Where previous 3D Mario games were about exploring spaces, Super Mario is in general about navigating to the end of the level. It's a mix of the styles of classic and 3D Mario.
It also makes more of an effort to integrate the 3D effect into gameplay. But the parts that don't really deal with that are a lot more effective than the ones that do.