mongodump default behaviour on 2.6.X was to ask for the password in interactive mode on the shell when you pass -p without arguments, now, since 3.0 it gives an error even though it clearly states that the old behaviour still works on the docs.

Command:

mongodump -h SERVER:PORT -d DATABASE -u USER -p --dumpDbUsersAndRoles -o OUTPUTDIR

On all 2.6.X it asked for the password on the terminal interactively and proceed to the dump.

Since 3.0.0 it gives the following error:

2015-03-23T15:00:33.996-0300	error parsing command line options: expected argument for flag `-p, --password'
2015-03-23T15:00:33.996-0300	try 'mongodump --help' for more information