Mario games have usually featured some kind of 2-player mode. Thing is, it normally stinks. Not so with Mario Galaxy! Shigeru Miyamoto believes that finally, after over 20 years of trying, Nintendo have cracked the secret of implementing a decent Mario multiplayer experience with the game's Co-Star mode (where a second player takes control of a little star and...rings bells and stuff). Or so he tells Nintendo boss Satoru Iwata, anyways:

Iwata: So, with the creation of the Co-Star mode do you feel that you've finally successfully solved this challenge that has spanned about 25 years since the birth of Mario Bros?

Miyamoto: Absolutely.

There you have it! Seems they've actually put quite a bit of work into the 2-player mode, stripping away some elements of the single-player game that were "too complex" and putting them in multiplayer instead. Miyamoto promises anyone controlling the star will be be playing "with a very strong sense of participation", and having listened to Ash bang on about how fantastic the game is, I'm inclined to believe the man.
The Old Issue of Two-Player Play [Nintendo UK]