Have you noticed that the Mario Kart series has been building to something?  Neither had I, but once you listen to Mario Kart 8 producer Hideki Konno explain how the Wii U sequel is intended to be the definitive Mario Kart game and the grand culmination of all of the elements in the series, perhaps you'll start to understand.  Konno discusses how the new anti-gravity gimmick expands the level of freedom the developers enjoy, how the shift to high definition visuals impacted the franchise, spotlights the Mario Kart TV feature which is an expansion of the old Mario Kart Channel for the Wii, and points out the expansive multiplayer and controller options.  There's a lot to cover in this Nintendo Developer Direct, but it's interesting stuff and highlights some of the design decisions that players take for granted when enjoying the finished product.  Just wait to you see Rainbow Road with designed anti-gravity in mind!  Say, if this Mario Kart is intended to be definitive, might that mean the next sequel will be completely different?  I suppose that's best left as a question for another day.