It would be really nice to have a preference when using 'Associated Application' such that it only matched to that option if there was a registered application for the given file type. This would allow us to specify a default editor for any type not already registered without getting that annoying windows popup, but still use the proper editor in the defined cases.
For me, this would mean something like:
Open 'Word' when I choose to edit a '.doc' file.
Open 'Notepad++' when I choose to edit a '.foobar' file.