There is a lot of new stuff coming up with SharePoint v3, so a brief overview of some of the new features:
- List item security
- Recycle Bin (user and admin level)
- Workflow! (using Windows Workflow Foundation)
- Other data source lookup columns
- UI improvements like better menu’s treeviews and configurable options
- RSS support
- (Custom) ContentTypes
- MasterPages (ASP.Net 2.0)
- Improved Definition structure
- Two-way Outlook integration
- Eventhandlers in every list
- Version history in every list
- Minor and major versioning
- Security on versions
- Email to lists
- Different authentication methods per domain (url)
A much longer list can be found here