Outlook got the most attention from Office XP to 2003. They implemented a nice three pane view, something that hasn't been copied overly well yet by others.

It also offers new security features, like not allowing executable attachments. Realisticially, it's just a pain to deal with though.

Entourage 2003 to 2004 (Office:Mac) also saw many changes. Seems Microsoft decided their office mail client needed to be updated.