$project allows insertion of new computed fields, however it doesn't seem to allow insertion of new literal fields.

{$project : {"newfield":"abc"}}
gives "errmsg" : "exception: field path references must be prefixed with a '$' (\"abc\""

It would be nice to not have to hack around it via:
{$project : {"newfield": {$add: ["abc",""]}}}

Possibly related to https://jira.mongodb.org/browse/SERVER-5762 and https://jira.mongodb.org/browse/SERVER-5782