here was what I did/saw
the first result message was confusing to me
the latter half it is actually the successful user addition, I thought it was showing me the spec of a user object
either better reporting or an error (and no op) in place of a warning should occur

> db.addUser("foo", "bar")
WARNING: This form of the addUser shell helper (that takes username, password, and readOnly boolean) is DEPRECATED. Use the form that takes a user object instead

{ "user" : "foo", "roles" : [ "dbOwner" ] }

> db.addUser(

{user:"foo", roles:["dbOwner"]}

)
2013-10-24T15:22:54.154-0400 Error: couldn't add user: Must provide a 'pwd' field for all user documents, except those with '$external' as the user's source db at src/mongo/shell/db.js:1005

> db.addUser(

{user:"hi", pwd:"asdf", roles:["dbOwner"]}

)

{ "user" : "hi", "roles" : [ "dbOwner" ] }