Several features and functionality that aren't available or supported in the hosting version:
- Exchange Management Console
- Public Folders
- Unified Messaging Server role
- Federation
- Global Address List Synchronization (GalSync)
- Business-to-Business features such as cross-premises message tracking and calendar sharing
- Information Rights Management (IRM)
- Managed folders
- Outlook 2003 support (EnableLegacyOutlook)
- Edge Transport Server role (more specifically EdgeSync support)
- In-place Upgrades (upgrading current Exchange 2003/2007 org to Exchange 2010)
- Resource forest topologies (separate Exchange forest and account forests)
- Parent-child domains
- Multiple AD sites (where there's a mix of hosted and non-hosted)
- SSO AD Trusts
- Dis-contiguous namespace
- Disjoint namespace