Nintendo Power has an interview in their latest issue with long time Nintendo veteran Takashi Tezuka, producer for New Super Mario Bros. A direct transcript isn't available, but N-Sider has quoted some of the more important bits. Most interesting to me is insight in exactly how the game is laid out:

The gameplay itself has a lot of replay value, I think. In addition, you won't be able to save your progress from level to level; you'll only be able to save when you reach the next world. So in that way, the game system is designed to challenge the player and encourage replay. Also, remember how in Super Mario 64 you needed to collect a certain number of stars to reach the next course? We're doing something similar to that. There are also similarities to Super Mario World in that different branches exist to different levels. By completing some task, you'll open a different branch.

It's funny -- he also talks about the Mario series evolving and becoming more difficult, and how he wants to get away from that, but they make it so you can't save except between worlds? And restricting when and where you can save on a portable device may not be the best plan.

Still, my main concern for the game is size (and along with that, length). One of the great things about the Mario series has been the sheer enormity of them, especially starting with Super Mario World. Branching paths and alternate exits are a good start, and the screeenshots that have been released show a good amount of variety. But Nintendo is playing secretive with everything nowadays, not just the Revolution, so we probably won't know many of the game's details until we have it on May 7th.

Source: N-Sider