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.
Here's the best way I can describe the perspective and level design in Super Mario 3D: it's like Paper Mario as an actual platformer, or like Final Fight. The camera is, for the most part, locked to an isometric perspective, and levels scroll mostly from left to right, with some room to move toward and away from the screen. So not completely flat. The levels do also wind in different directions: straight up, or into the screen like a true 3D platformer -- but never in the wide open worlds of a Super Mario 64. It's much more linear. There's a path. In that way, it feels much more like the NES/SNES Marios than like one of the 3D iterations.
For the most part, though, I like the level design conventions: the linear design puts more of an emphasis on platforming and enemy evasion rather than exploration, and that's been Mario's strong suit. Another strong Mario suit: the tanooki costume, which makes its first appearance in this game since Super Mario Bros. 3. It's a nostalgic favorite, but if the things you like about the tanooki suit are flight and turning into a statue ... you're not going to like this one. You can whip your tail, and you can slow your fall, but you can't fly, something Nintendo attributed to the difficulties of implementing flight in the game's perspective (during the Nintendo developer roundtable).
There's kind of a Super Mario Bros. 3 theme throughout. One of the levels I played was an airship, almost exactly like the auto-scrolling airships from Mario 3. There were Bullet Bills shooting all over the place, Rocky Wrenches popping up to throw spinning wrenches, and, playing to the 3D screen, huge spiky columns firing straight out of the screen. And then, at the end, I fought a 3D-ized version of the "classic" Mario 3 midboss that I always forget exists, Boom Boom. Now, instead of just moving back and forth in a straight line, he chases you around a square arena. As a result, he's way more difficult!
It was this airship level, and a classic subterranean level like Super Mario Bros.'s 1-2, that won me over. Clearly, the daytime, outdoor levels in which the stage winds around in all directions, with lots of trick platforms, are the ones meant to impress, but the ones that keep it simpler were the more enjoyable experiences, and the ones that give me the most hope about this game.
Well, that's not entirely true. What gives me the most hope was the Zelda-inspired stage Nintendo showed on video during the developer conference. Super Mario 3D has a stage that emulates the room-to-room design, top-down perspective, and basic room layout of a dungeon from the first Legend of Zelda. There are even torches that Mario has to light with fireballs to open switches. I want that game.