Overall how does the track design in this game stack up?
Overall, really well. Probably the best selection yet.
I'm not taken by the underwater driving, so the tracks that have underwater sections are my least favourite. But there is a good selection of fast, SNES-style tracks that show of how lovely the base handling in the game is. Some tracks are 3 separate sections, rather than three laps, and they're all great additions.
The Wuhu Island ones could have been taken from Ridge Racer. The Wii Music track is like a Nintendo version of a Wipeout HD track. Despite being mostly underwater, Wario's Galleon is a Nintendofied version of the Daytona 2 theme park track. Rainbow Road is glorious.
Of the classic tracks, the SNES ones are great and well suited to he toned down racing in MK7. The N64 ones even more dull than normal, and the DS ones look much prettier. SNES Rainbow Road is beautiful and an improvement over the original.