Volume label mru function would be helpful for repetitive volume names. It can remember user specified number of items. I mostly use like this labels "MY MP3s Vol. XXX". Also on label section there can be a favorite labels button for predefined labels.
Another suggestion about it: Automatic incremental number if label has numbers. Example I burn "MY MP3s Vol. 151" then next time imgburn launched or new project added if I select "MY MP3s Vol. 151" from mru then label automatically changes to "MY MP3s Vol. 152". I know it's a bit futuristic function but in this way users don't have to remember volume numbers.
Most combobox not work as you described, even most web combobox. I haven't experienced any not sure what's in that box situation so far. Also not adding without enter or + function doesn't help to sure what's it.
Hi. Source folder selection via drop-down needs additional click on + button. Selecting via drop-down mean I select it so I think no need further clicking on +.