Real PIM functionality
All aspects of PIM should sync with Outlook. This includes Tasks and Notes. Calendars should function like they do in Outlook. Support for Outlook color coding would be nice too. The ability to make an appointment on an iPhone Private would be nice. Same with the ability to select categories.
Gee, why not the ability to send meeting invitations from the iPhone? This is standard on most Windows Mobile devices. Should be here too since this is the major competitor. This feature alone could cause me to drop my iPhone in favor of a Windows Mobile device.