What are the differences when playing as Luigi?

Alright, I've done some further digging, and these are the differences I've found:

• Uses Air when spinning underwater
• Has less traction, so he slides and skids
• Jumps farther and higher; runs faster
• (perhaps related to the above) knocked back slightly farther when hit, which equates to a longer post-hit stun
• The cosmic Luigi challenge runs take advantage of Luigi's physics, which may mean separate routes than cosmic Mario
• In addition to the above, some galaxies have slightly altered levels, such as the "Luigi meets Luigi" star in Toytime Galaxy, which changes from the 8-bit Mario planet to the backside, which features an 8-bit Luigi (with the change in colors coming from a change in the platform "pixels")
• Mario gets 1-ups from Princess Peach's letters, but Luigi gets 20 1-ups instead (It still says "Dear Mario" Poor guy can't catch a break)

Besides what Raven added, I know that Luigi's cosmic double challenges are significantly more difficult than Mario's. Also, some of the levels change slightly (such as the 2d Mario one, becomes a 2d Luigi one).