As someone who has taken an interest in game design, there were a few things I found interesting in Super Mario Galaxy. Mind, the last game I played was 64, so some of these may not be novel to this release.
The Constrained Path – A Return to Form
Many levels, especially the early ones have a very linear layout. Very often you are getting launched from tiny-planet to tiny-planet. There is often freedom of movement on each planet, but movement between them is limited by launch stars. This retreats a bit from the freedom of 3d and harkens back to the linear scrolling levels of the first games.
Levels of Scale
Flying between planets has an effect almost like levels-within-levels. The level structure has a quality of Levels of Scale – one of Christopher Alexander’s 15 properties of life. There is the largest structure of six worlds, each with 2 or 3 major worlds and a few extras. Each of the major worlds has about 6 stars available, which may (or may not) reuse the same basic level geometry. Within each level, there are very often multiple sub-challenges, if not physically separate planetoids.
Variation of Challenge
There are several different types of levels. Major classes include timed, freeform exploration, boss fights, auto-scrolling, ball-rolling, ray-surfing. One of the great wonders of the game is how many games are contained within it – whole mechanics that would have been sufficient for a single game in ages past now supply 3, 2, even just 1 level (red star comes to mind)
One intriguing feature is the players ability to get two different types of resources from goobmas, and probably other kinds of enemies. Spin/kick them an they turn into star bits, which serve several purposes, including adding to extra lives. Stomp them and get a coin, which replenishes your health (they can also add up to lives, but are lost every time you die, making it much harder to collect enough.)
(Ideas 2008-02-22, mostly written 2008-07-06)