I'm wondering if given the excitement about the dual nature, we might be losing part of the intention of IObservable.
First of all, I'm thinking there might be a bit of a mismatch in the duality anyway. Primarily, IEnumerator and IEnumerable have a many-to-one relationship, whereas IObserver and IObservable havea many-to-many relationship, right? The deal is that IEnumerator has to have intimate knowledge about the structure of its IEnumerable that the app doesn't necessarily have; they're tightly bound. Whereas IObserver and IObservable are completely unaware of each others' internals.
I think the dual of IEnumerable would be something more like ILoadable, where you call ILoadable.GetLoader(), which then allows you to load values into an "array" or whatever, using Loader.MoveNext(bool) and Loader.SetValue(T). I certainly think the intention of IObservable is more valuable than any ILoadable concept (though ILoadable might be an interesting way to explore inserts and updates), so I wonder if the focus on duality is perhaps the wrong way to go about it.
For example, I'm not sure I get the point of the OnError method. First of all, isn't there a duality mismatch here anyway—given that IObserver.OnNext(T) could also throw an exception, then to preserve absolute duality going the opposite direction, shouldn't we require that OnError also be a part of IEnumerator? But beyond that, it just doesn't seem to fit. Why would the Observable need to tell all Observers that there was some exception downstream? Just like an iterator doesn't need to know that there's an exception occurring in the app that uses it. So I think the focus on preserving duality really mucks up the intention of IObservable in this case.
Also, I think if duality was that important to the concept of IObservable and IEnumerable, then the dual of (for example) IList would be even more powerful. But wouldn't the dual of (for example) IList.Set(int, T) be IListDual.Get(out int, out T)? Is that useful in some fashion? I don't see anything, though I'd love to hear from anyone with an idea how it could be powerful.