The AsQueryable<T> extension method is currently declared as:

public static IQueryable<T> AsQueryable<T>(this MongoCollection collection)

and this requires you to provide the <T> argument explicitly. This is not a bad thing in a schema free database where the same collection can contain different kinds of documents. In practice though, it is common for a collection to contain only a single kind of document, and adding the following new overload of AsQueryable<T> would allow <T> to be deduced from the collection:

public static IQueryable<T> AsQueryable<T>(this MongoCollection<T> collection)

I can't see any reason why the two overloads can't coexist.