It would be nice if we could, as an alternative to drag-and-drop or dialog box config on the mashup editor, be able to directly edit the XML representation of the mashup in the Composer environment.
When a mashup gets a bit complex or has several services, it gets difficult and cumbersome to drag and drop. The binding configuration screens can be time consuming as well. Additionally, if we have to refactor a service into another Thing, there's no easy way to replace that in a mashup without having to reconfigure all the bindings and events to/from that service. To get around that, we sometimes export the mashup, make changes in the XML file, and reimport to save time.
Having the XML representation in another tab (maybe near the custom css tab) would alleviate a lot of headaches we have with the mashup builder. Many other WYSIWYG type editors have this functionality.