Flag which navigation properties have been loaded
It's currently not possible to determine if a navigation property has been loaded.
For example a order.orderLines. There is no way of knowing whether orderLines is empty because the order has no lines or that the navigation property has been loaded.
This is a real pain when you have business logic that require the object graph to be loaded. This leads you to implement boiler plate data access related code to manually track which navigation property has been loaded - something that the breeze framework should be doing.
This feature has been asked for on SO here: http://stackoverflow.com/questions/17798514/any-way-to-tell-if-a-breeze-navigation-property-has-already-been-loaded