It would be useful to get support for the $comment meta operator in the C# driver. This would allow users to tag their queries with a comment that shows up in system.profile if the query is slow (or if all queries are being profiled).
This could be something like:
collection.FindAll().SetComment("Tag my query").ToList()
and for the LINQ version:
collection.AsQueryable().Where(o => o.Field == 1).WithComment("Tag my query").ToList()