They're really going after want I eventually wanted out of 3D Mario: a total mind-fuhk.
Mario 64 was great and all as it implemented 3D. But as time went on, it and many more 3D games (FPSs, notoriously) are still dominated by traversal on 2D planes/floors. It's a natural setting, afterall, but it took so long for a high-profile game to tackle the task of expanding the notion of the "playing field" further.
I'd argue the most exhilirating moments of Mario 64 involve the Wing Cap, thanks to its sharply contrasted sense of freedom (and danger?) from the typical platforming -- aspects Mario Sunshine severly lacked (way too terrestrial, not enough time spent grasping for the heavens)