Just a thought but I think you could do something similar using a group of playlists. One playlist per person. There's a page on the properties panel for selecting which playlists a file is contained in which uses checkboxes.
But I agree, conceptually the List data type is very powerful, but more work needs to be done on the interface to reveal the power. Basically what's needed is for a small [...] button next to List fields, that pops up a dialog box with the following controls:
Category: A combobox that allows a named set of keywords to be selected from a predefined list (such as People or Fruit). This would be better than having just one global keywords list.
Categories...: A button that opens another dialog box that allows new categories to be created and keywords added to them. Eg, People={Me;Mum;Dad} Fruit={Apple;Banana;Cranberry}.
Keywords: A listbox of the keywords defined for the selected category, with checkboxes beside each item. Checked if all files in the selection have this keyword, gray checkmark if at least one of the selected files have the keyword, or no checkmark if no files have the keyword. Click checkboxes to toggle between the three states.
OK: Button to update fields of selected files. If any checkmarks are still gray, that keyword will only appear in those files that originally included them. Otherwise that keyword will be added to or removed from all selected files.
I think this would provide many possibilities for quick categorization of files.
Another possibility to make things easier without too much extra effort would be to support the following syntax for values of List field when several files are selected (ie when "<varies>" is shown):
+Pop +Rock -Blues
Rather than setting the field of each selected file to "+Pop +Rock -Blues" it should add "Pop" and "Rock" to the list of delimited fields to each file if that value is not already present, and delete "Blues" if it is. Another useful idea would be to add a character that indicates you want this field for each file to be resorted so that, eg, "Pop; Rock; Blues" becomes "Blues; Pop; Rock".