Unlockables galore, varied and (sometimes) challenging maps and most of all fun. I have a lot of priors in retrospect with Mario Kart Wii, but can still go back to it occasionally and find it playable. It's a big, bright, colorful racer where almost anyone can pick up and be competing at a low level within a matter of minutes or hours depending on how it sticks.
Much of what holds it back for me still plague the series in 8 Deluxe; track texture and grippiness do not "feel" right, customization for the karts and bikes is incredibly sparse etc. But, I can not recommend the game enough.