Implement True Tagging
Outlook supports categories but full tag support would be so much more flexible. Need to be able to quickly and easily create tags on all Outlook objects and quickly create views and search folders based on multiple tags. It needs to be very easy and fast to tag an email. Maybe a hotkey or button where you can enter custom keywords auto-completing existing ones, etc.
Tagging each incoming mail along with some notes may just be most useful - definitely for me.