I got the game around midnight last night and played long enough to do one run through each cup with 100cc engines. So, I have all(?) of the tracks unlocked. Early impression: this is the most robust Mario Kart I've played since MK64. 👍 👍
My only criticism is the overly simplistic menus, with limited singleplayer options. Music/sound volume control would be nice, if I wanted to listen to something else from my laptop. MKTV is nicely implemented -- it saves full replays of your last six races without requiring a prompt -- but I don't think I'll be uploading anything. So far, I've blasted blue shells with the Super Horn at least as many times as I've been hit by one. Against the AI (no online yet), I've felt refreshingly undisturbed, but they're no pushover either.
Given my affinity for racing (naturally), it's quite nice to finally have something with that sort of replay value on Wii U.