So I got a WiiU and this game about a week ago. Been loving it so far. Shy Guy is easily my favorite character in this game.
My favorites among the 16 new courses.
Thwomp Ruins
Spoiler:
Sunshine Airport
Spoiler:
Shy Guy Falls
Spoiler:
Cloudtop Cruise
Spoiler:
Twisted Mansion
Spoiler:
Dolphin Shoals
Spoiler:
Electrodrome
Spoiler:
Bowser's Castle
Spoiler:
I recommend watching these videos in higher quality settings, because the lower qualities don't really bring out how good this game actually looks, obviously.
This game has a lot of little touches to it that I like. The music get's a little extra flair whenever you're in 1st place, the giant eel from Super Mario 64 let's you ride down his back in the Dolphin Shoals course, and the karts and bikes are now much more balanced with each other than they were in Mario Kart Wii. The new items are cool too.
I'll post all my favorite retro courses some other time.