I really liked operator overloading in version 1.0.2005.1

But for example why it is required to type :

bucket.Relations.Add(CustomerEntity.Relations.OperationalSiteEntityUsingOperationalSiteID)
                .CustomFilter = new PredicateExpression(OperationalSiteFields.OperationalSiteID == OrderFields.OperationalSiteID);

instead of

bucket.Relations.Add(CustomerEntity.Relations.OperationalSiteEntityUsingOperationalSiteID)
                .CustomFilter = (OperationalSiteFields.OperationalSiteID == orderFields.OperationalSiteID);

This generates an error that Predicate could not be converted to PredicateExpression

Because the operator creates an IPredicate instance, (like FieldCompareValuePredicate), and CustomFilter requires a PredicateExpression, which is a subtype of Predicate.

Otis wrote:

Because the operator creates an IPredicate instance, (like FieldCompareValuePredicate), and CustomFilter requires a PredicateExpression, which is a subtype of Predicate.

I know that. Could Custom filter require Predicate?

It's a property, so it can just have 1 type. To make it a predicate gives problems for code which reads this property. (3rd party code), and that would therefore break. I don't see an advantage in making this property of IPredicate type.