Koopa Troopa is a racer for 3 times now (Super Mario Kart, Mario Kart Wii, and Mario Kart 7).
Lakitu is a new racer (Mario Kart 7).
Boo and Bob-Omb are items, and only Pink Bob-Omb and Pink Boo can be a Character, if you want (note that red Lakitu is a racer, while the green one can only be seen at the Laps and such).
Goomba is an obstacle, so only Paragoomba can be a Character.
I really don't mind as long as Toad is in the game. I think the generic enemies are sometimes fun to use, and besides Koopa was one of the original Mario Kart characters so I think he somewhat deserves his place (seeing how many unimportant characters are being added into the newer Mario Kart games).