It differs from Robotlegs in that there is less leg work (pun) in setting up the command roster. Commands automatically register themselves to their parents, and their Events also acts as their mediators/models. This reduces redundant code significantly.

As a fellow framework developer, I would like to get your feedback as well.

“Robotlegs actively defines a PureMVC style MVC structure to your application.”

Not exclusively. The example mvcs.* implementation does, but the idea is that you can create other architectures (contexts), as long as they promise to satisfy dependencies, and then run them along side each other in one instance of the AVM (with no unintentional cross-talk). So, while the current mvcs implementation is PureMVC-style, future architectures need not be.

“You are extending framework classes with your actors instead of using plain-old-actionscript objects.”

Only if you want to. For the most part, the mvcs implementations just provide some oft-needed dependencies (references to the context's injector, event dispatcher etc), and some convenience methods (though I am skeptical of these): you could skip the inheritance and just add the dependencies to plain old objects by hand, as and when you need them. Well,.. that's the idea anyway – how reasonable/useful that is I'm not sure yet.