Characters are not so much, It is difficult to put stickers and decorations on your player.
Karts you can even make your own from pure scratch. Grab some materials shape it how you want and save it as a kart, it won't look as great as a pre-made object however
Track is where the game will shine. It wipes MNR in the dust. Customize Weapons, Physics (Kart speed, Jump, Disable or enable things like Drift, shields etc) and even the gameplay. Make Battle arenas or even mission or minigame levels. The editor is like LBP's and can be used to make non racing games. I've seen First Person Shooters in the beta
Little King of LBP Level Recordings http://www.youtube.com/user/JHKthree