Native support for GUID's would be nice. Right now ObjectId's are the only (native) way to manage unique identifiers. But in some projects ot may not be possible to use ObjectId's - either due to legacy code or other constrains.

My pet reason for not using ObjectId's is that they leak persistency concerns into my domain entities. By using GUID's I avoid making the code dependent directly on a class defined by MongoDB (using the Norm C# driver).

It is possible to use GUID's but they are stored as binary blobs and as such one cannot read and use the values through the shell while debugging.