I played a little of Kirby Air Ride for the first time at PAX and loved it. And it showed me concretely that a kart racer doesn't have to be exactly like Mario Kart "but with a few new bells and whistles" to be exciting, new and fun. The game was way faster than any Mario Kart game, with track designs reminiscent sometimes of F-Zero, a cool power-up system, and did gliding mechanics way before MK did. Not to mention all the unlockables we didn't get to. And I haven't played that single-player mode lots of people I know rave about, either.
Mario Kart is fun, yes. But more could be done with it, and has. In other franchises.