While it's possible via combined use of $objectToArray and $filter with $map, it would be nice to have a simple expression to get value of particular field of an object:
This will allow easily plucking out a value when the field name is stored in another part of the same document, or when it has some funny character that precludes it being used as subfield name (like leading "$") as in SERVER-30365 and
SERVER-14466 or embedded '.' when querying oplog or system.profile collections.
Second argument could be optional and default to "$$ROOT".