If you use the ToString function on a DateTime object in an expression tree for an aggregation query without specifying a format string, it will produce an ArgumentNullException. It would also be nice to add support for the shortcut functions such as ToLongDateString and the various single-character format options supported by the .NET Framework.

I've also noticed that MongoDB uses a different format of date/time format strings than the .NET Framework, and there is no translation between them. I'm not sure whether this is a bug or a feature.