The write concern w:0 has special meaning for write commands - suppress all output, since the client doesn't care. This currently interacts badly with wrapping the returned response in the shell, for example:

Bulk results probably have the same issues.

> db.coll.setWriteConcern({ w : 0 })
> db.coll.insert({ _id : 0, hello : "world" })
WriteResult({ "nInserted" : NaN })