I often reuse the same queries in a Layer Properties' Definition Query. It would be nice to be able to save these various queries as "custom/predefined" queries so I could go back and select them from a drop-down list. This would probably also be handy in other Query dialog boxes.
PS. I currently save the SQL statements in the Layer Properties' Description (under the General tab) so I can cut and paste them back and forth... but a "Notes" text box instead would eliminate conflicts with the Metadata information.