I would just appreciate the lack of blue shell.
Anyways, I've played all (except the SNES) the games, and my hands-down favourite has to be Double Dash. The co-op was not really needed, but the drifting, tight tracks, and actual skill it took to use items always left my friends in the dust.
Definitly my favourtie gamecube game.
I hope they re-implement some sort of character specific item function like in double dash though.
I sense that the under-water racing will just slow things down. As with almost all underwater segments, they simply slow down the game and reduce your traction.
The air segemtns seem like they would be more fun, especially if you have to dodge obstcles on your descent.
Customizable karts could provide some degree of driving style that could differ from person to person, and if they make enough design choices, it could provide a layer of depth (depth in a Mario Kart game!?) to multiplayer.
Overall I'm excited for this, and if it can replecate the multiplayer experience of Double Dash, I'd be really, really happy.