The interactions are not clear between mongorestore --w (which is the way to do it in MongoDB 2.6 and earlier):

http://docs.mongodb.org/manual/reference/program/mongorestore/#cmdoption--w

and mongorestore --writeConcern (which is new in 3.0):

http://docs.mongodb.org/manual/reference/program/mongorestore/#cmdoption--writeConcern

In particular, --w defaults to 0 but --writeConcern defaults to "majority". Apparently --writeConcern "wins" in 3.0, but this isn't clear.

It's also not clear what happens if both are specified, or if there is any preference between the two. Presumably "--w N" is (now) merely a shortcut for "--writeConcern '{ w: N }'"? And the --w default is antiquated and no longer applies?