Possible Clues:

• Parts of the speech have nothing in the good sisters
• Flotsam and jetsam, for example?
• Things of all sorts
• Bell, book and candle
• Nothing in the good sisters for such names
• Verb preceders
• Bush and Gore, e.g.
• They can be used to refer to persons, places or things
• Words used as names of places or things
• Can't be proper if they are common