One reason why I'd reason there not being dragon flying is the nature of the top-down cRPG map. While there's more world detail presumably due to being able to play a more "Over the shoulder" perspective like Swen enjoys doing in the gameplay demos, we also know that there are acts and each act has its own area, similar to DOS2. So if you were able to fly like that, the world would feel pretty small pretty fast, and there would need to be additional mapping design around the edges of the map to indicate you can't fly further than that, but I don't see any way that can be done without being immersion breaking.
