Larry asks:
"I can set a Flag for a Contact, including a due date for that Flag. However, short of opening the Contact, I can't see to display the due date. It would be great if I could include it on the folder view, be able to filter / sort by it, etc."

To get the view Larry wants, all he needs to do is create a custom view and use the Due By field from All Mail fields dropdown, not the one in All Appointment fields or Due Date from All Tasks fields, both are for a different Due By field.

You can use any field from any item type in any custom view - some will display useful information, others will display nothing. If you aren't sure which field might display the information you need, drag several fields to the view and remove the ones that don't display useful information. If a field is available in more than one item type, try them all - if a field is not listed under All [folder type] fields, it means it's already in the view using a different display name, such as Organizer and From fields.