http://www.gamasutra.com/php-bin/news_index.php?story=16386
The evolution of Mario gameplay via an interview with Super Mario Galaxy game director Yoshiaki Koizumi (surprisingly interesting): “There are issues with 3D action, including problematic depth perception, getting lost, and even getting motion sickness. Those phrases, Koizumi said, are the keys the team worries the most about when creating a game in a 3D world.”