I would love to see this improvement make it into v2.0.

Consider this basic class:

class Book
{
// ....
public List<Ratings> Ratings

}

And this query:

var allGoodRatings =
from b in mongo.Books
from r in b.Ratings
where r.Stars >= 4
orderby r.Stars descending
select r;

AFAIK, this crashes with SelectMany not supported. It would be super easy to implement in the drive and as long as the second set is within each document it references, this could happen on the server and we don't have to resort to hacks where we double query it once on the db and then again on the client.