Since editing XML workflow files is very closed to programming, it's necessary that you provide a web editor so that administrators can modify it easily.
Today, it's too difficult to edit the XML workflow file and to test it.
The editor should offer to the user a proper interface to guide him across the creation of workflow steps and workflow actions, without the need to know JIRA internal classes names or other internal stuffs.
The editor should always generate valid XML files.
The user should also be authorised to manually modify the XML file and the editor should be able to read and analyse manual changes.

There's no need to have a complex graphical view of the workflow, but only wizards that guide the user across the creation of steps and actions.