Multiple menu item selection
Every so often you'd like to select several menu items and get slightly annoyed that the menu keeps closing with every click. Think of showing a bunch of email attributes (From, Subject, When, Status) in Tracker for example.
How about the possibility to do it all in one go: left clicking an item shows the current behaviour, right clicking sends the BMessage of the menu item but keeps the menu open. So you'll see any changes live, not only after the menu is closed.