I think charging for quality map packs is fine. Cosmetic items are also fine, although I pity the fool who buys them.
Anything which changes the balance of the game in multiplayer mode is a no no though, and it would be enough to stop me from buying said game. An example would be new weapons (as Battlefield:Bad Company is rumoured to have).
My Mario Kart Wii friend code: 2707-1866-0957