Sub todo's for one thing. I use this for all my project management.
Birthdays and Aniversaries can be added to the calendar automatically from the address book. I actually helped with this one a little bit.
Todo's are shown on the caneldar as well, no more having to flip back and forth between the calendar and todo just to see everything.
No .pst. no 2G limit, no running scanpst to fix it.
Integration of mail and news, no more crappy outlook express.
Linking of files to calendar/task entries. I had to do this myself by adding another tab to the default form in Outlook.
The only thing I miss is being able to assign categories to files on the file system but OL2003 did away with the file system plug in anyway so not that point in outlooks favor is no more. I'm learning to use the metadata anyway which is more portable.
Korganizer became the killer app which helped me make the switch to linux.