That's a pretty interesting idea...but looking at the action editor, it seems like most of the things you can do are not as suitable for a drag-and-drop implementation as the ones you mentioned. For example, setting the context or project seems like an obvious candidate, but if your list of projects or contexts doesn't all fit on the screen at once, do you have to scroll it into view first (never mind that such a list isn't showing when you are looking at items in the Inbox)? If you have more than a few dozen projects, scrolling through the list can take a while, and if you've got things nested in folders, you have to stop and drill into them. On the other hand, if you use the smart matching for project and context names, you can set the value to nearly any project or context name with just a few key taps. Try using just the first letter of each word in the title, for example. With a little practice, I think you'll find this approach is faster.
Also, from a UI design point of view, I think it is easier to learn the application if everything works in the same way instead of "well, to set the project, you tap a button in the action editor, but to make it into a project, you drag it over here." If you could come up with a drag and drop scheme that handled most of the operations done with the action editor now, that would be more compelling. Often, one isn't changing just one thing about an action or project, and it would be strange to have to do some of the manipulations by dragging and dropping and some via the action editor.
There's definitely room for improvement in the drag and drop editing of actions in a given project right now, which is where it seems to be the most natural fit. Regardless of whether you agree with me about the utility of drag and drop in the UI, questions and suggestions for Omni are best sent directly to them with the Contact Omni menu item. That gets them into their tracking system, whereas a post to the forum is not guaranteed to do so.