Some points - since you are hosting this in Eclipse, it seems that you are focusing on developers. Most graphic designers won't touch Eclipse. Also most developers once they experience another IDE (IntelliJ, for example) find it harder to return.
Maybe nitpicky but when I drag an object like a container or button into the screen canvas, it messes with my flow when I get an immediate edit box for the text attribute. I'd rather double-click to edit.
Comparing with Balsamiq, one thing that I don't like about Balsamiq is you can't yet have named groups of items (e.g. all the navigation elements). Being able to have a reusable user-defined library of custom groups - e.g. new "Edit screen for editing Foo in module X" would differentiate mockup tools from the spawn of Visio.
Grouping logically would be the difference between bitmap type editing and Illustrator-type editing. It would be nice to be able to have an outline tree view of elements organized in a hierarchy by UX intent.