Nice interview with Nintendo game designer Yoshiaki Koizumi, particularly the bits about shifting from 2-D to 3-D Mario games and Mario Galaxy. The bulk of the gameplay in Galaxy takes place on spherical surfaces:
He explained that no matter how large you make the playing field, if you walk long enough you will run into a wall, and that will make you turn around, which makes the camera turn around and runs the risk of making the player lost. With a sphere, Mario can run all he wants without falling or hitting a wall... a useful concept for getting players totally absorbed in the moment. Koizumi added that the best thing about spherical worlds is the "unity of surface," and the "connectedness." Neither will the player get lost easily, or need to adjust the camera - by using spheres, Koizumi said, they had created a game field that never ended.
They also talk about the Galaxy's two-player (well, 1.5-player really) feature, which is a really nice way of getting a second passive player involved in what is essentially a one-player game. (via snarkmarket)