MOSS 2007 has a lot of features that weren't there in SPS 2003 or that didn't work that well in SPS 2003. Having a list of these features can really help to make the decision to migrate to MOSS 2007.
Here on Office Online you can find a great comparison between SPS 2003 and MOSS 2007 features.
Some things of the great new features in MOSS 207 are:
- Social Networking web part (especially when combined with Knowledge Network)
- Mobile device support
- Easy customizeable navigation
- Aggregation web parts like the Content Query web part to roll up data and documents
- Integration with Outlook 2007 and the ability to take documents offline
- Showing business data in SharePoint with the Business Data Catalog
- And lots and lots more... ;-)