I would also like to be able to "stage" a backup: figure out what needs to be transmitted and then create the data files that need to be transmitted without actually immediately transmitting it.

That would allow me to do things like backup my laptop to another computer in my house that can upload the files over my slow connection overnight when my laptop isn't on; and to let me bring the backup files to a place (work/university/library) with a fast connection so large backups don't take days or weeks (especially initial backup).