Stores need to be extendable, and the Mixin concept (as used in React Elements) are a very useful way to do it. In this way, one can easily implement Marty's "State Sources" concept. I released a fork of Fluxxor a while ago with Mixin support [1] but it is unlikely to make it until 2.0.
The other nice parts to my eyes are:
State Mixins - sort of a hybrid between Fluxxor's StoreWatchMixin and Fluxxor-autobind [2]
Constants - This is a new one, I like it. Will have to play with it to see if it truly solves any problems; I've found that when I need to know what actions are available, I scan my actions files, simple as that.
Marty seems a little more "batteries included" than the other Flux implementations on the scene. This is a good thing, so long as it is easy to extend. Unfortunately I still don't see Mixins in Marty Stores either. In my opinion they are required for non-trivial apps.