When it comes to customization, the communities are actually very limited. It's not like in the Super Smash Bros. series where you can electively turn on or off whatever item you choose and adjust their frequency. You start with one long list of the different items, characters and kart parts in the game, and you choose one of these as a specification.
In other words, you can have a green shell only race or a Luigi only race or a standard kart frame race. You can't specify more than one at a time, so wave bye-bye to your dreams having a no Blue Shell, mushroom wheel race. That part of the communities I found a bit disappointing, though perhaps it will make more sense in the context of the series when I actually have the time to sit down and toy with it as much as I want.