I'm not too bothered with the character roster in this game too much, though i've always been fond of Birdo as no one seems to show her (or even him 0.o) some love.
The only thing i'm interesting in is the whole business with "customizing" your karts. It's obvious this will affect your handling, drift and pretty much overall preformance on certain tracks, but i want to know whether you'll be able to unlock a wide range of parts to give players alot of variaty so we don't end up seeing the same kart layout being used online (similar too how a majority of competitive players use Funky/Daisy due to their good stats). Plus it's an overall nice gimmick for the game, similar to F-Zero GX (maybe one day they'll remake it Wii U).
Once this game is released i'm pretty sure 3DS sales will sky-rocket.