A feature I would love to see is the ability to drag a tab, and drop it in a JavaScript application where it can be capture by the web page. For example, drag a tab and drop it on to a google docs document.
Going the other way, if my application has an iFrame, I should be able to drag that iFrame and drop it on the tab bar and it will open that iFrame as a new tab.
If an application knows about the the content of the iFrame, it could then (in theory) control the displayed content.
A trivial example would be, I have a contact management web application. A user could open the weather network in a tab, then drag that tab to my application. My application detects this and not only displays the iFrame with the weather network, but controls it so that when I'm looking at a different contact, the location in the iFrame changes. Dragging the iFrame to the tab bar, opens (and dis-associates) the iFrame URL in a new tab.