Mario Kart just isn't a system purchasing title IMO. Maybe i'm too old. How long can you possibly spend playing mario kart, not to mention is all rather redundant at this point. How about a little innovation.
I can still fire up mario kart for N64 and have a blast. Same goes with the DS mario kart. The Wii Mario Kart was amazing, as well. Actually every Mario kart I have played I loved. They all did something different, but better.
The innovation will be there in the new Wii U version. 5 player local kart races; enhanced online; tougher tracks; anti-gravity; gliders, bikes, underwater; more customization options.
Even if this was just "Mario Kart Wii HD" i would be fine with it because Mario Kart Wii is outstanding, especially the online play (even though Wiis online system sucks). Hell, Mariokart Wii sold a shit load; so I do believe it's a system seller and realistically the best one Nintendo has to come since it appeals to everyone.