Any per-account limitations I could enforce would just cause people to register multiple accounts to circumvent them. Any per-account perks for players, like easier item exchange or easy switching between characters, could be substituted by adding a "link character" feature. You log into characer A, click on "link account" and enter the name and password of character B. Now the two characters - as well as any characters already linked to them, are then considered to be owned by the same player. The player can then switch between them without needing a password and can easily exchange items between them.
This would allow to have "lazy registration" like, for example, in forumwarz. The player can play the tutorial without registration and is prompted to register after completing it. This results in a really low entry barrier.