I think the selection of retro courses may be the best yet in this game. It was kind of hit-and-miss in the previous two games - this time, there's a lot of my favourites in there, and also less SNES & GBA tracks. No disrespect to Super Mario Kart and Super Circuit, it's just that their tracks are very flat and not very dynamic by today's standards.
Not only that, but there also seems to be more changes to the retro courses. Obviously, they're being adapted for the glider and propeller, but there were a few other things. I don't recall the Boos popping out of the paintings in the Luigi's Mansion track on DS.