First class, generic properties (see Rx) and use of first class observable events (IObservable).
It's a pity that WPF's property and event systems are island in the middle of a bunch of other great runtime-level and library-level enhancements to key concepts like generic properties and observable events. The snippets required for DPs and such are the best testimonials of this archeology.
Johann Dirry commented
It should be possible to bind WPF components to IObservable interface properties directly.
* thin MVVM View Models without implementing INotifyPropertyChanged.
* increased developer productivity because it is less code
* reduced bugs, because of simpler ViewModels
* better readable code
* separation of concerns