OL2K tracks the last Calendar folders you opened from other users' mailboxes, making it easier to open them again. Internet group scheduling using the iCalendar standard. I have not seen this working yet, but it is supposed to allow you to set up meetings with non-Exchange Server users. Invite new attendees to a meeting or remove someone from the attendees list without the need to send a meeting update message to everyone else involved in the meeting. The main reason for keeping that Personal Address Book around is going to disappear.
Activities tab to show items related to contacts.
The Journal tab on the Contact form goes away, replaced by an Activities tab that can show not just Journal entries related to a contact, but any type of item related to a contact, no matter what folder it's in. You also can manually link any item to any contact. Improved mail merge with Word. You can select contacts and initiate a merge from within Outlook. No more marking each folder in your mailbox for offline use.