I know I've talked about how much I liked Outlook 2003 in the past... but it wasn't until I had to roll back to Outlook XP for awhile until I realized what an improvement it was.
The colored flags that can be set directly in the column (as opposed to setting via a dialog) are absolutely awesome. I can't believe I managed to stay reasonably organized without the new search folders.
And those desktop alerts are just really cool. Perhaps the best thing about them is that they are non-intrusive - they are partially transparent and don't take focus.
After talking to some who went to PDC and watching some of the videos on MSDN, it sounds like we're going to see a whole lot more of this in Longhorn. Except it will be pervasive for all aspects of the user experience, and not exlusive to a single application (Outlook in this case).
I'm suddenly very excited for Longhorn if this attention to UI details continues - and all signs seem to indicate that it will.