In the older Samus C# driver supported having non-constant boolean expression in the LINQ query. For example this is a valid expression in the Samus driver...

//Get the first post having mismatched character count of its body length....
var post = collection.Linq().First(x => x.CharCount != x.Body.Length);

However in the official driver this is an invalid expression and throws an ArgumentException...

//Get the first post having mismatched character count of its body length....
var post = collection.AsQueryable().First(x => x.CharCount != x.Body.Length);

In his response, Craig G. Wilson indicates that the Samus driver is transforming the binary expression of a LINQ call into a MapReduce. It would seem that this is what the official driver should do as well. The official driver seems limiting and non-intuitive if it is going to allow LINQ queries. It clearly should support the complete complement of possible binary expressions not just a constant r-expression of a binary expression.

Here's a link to the question on the mongo-charp forum ...
https://groups.google.com/forum/#!topic/mongodb-csharp/XAz-TiCctqE