1. Windows Hyper-V hypervisor-based, hardware-accelerated server virtualization supports 64-bit guests, VM snapshots, VM relocation, access to offline disk images, and reservation of physical peripherals and CPU cores for exclusive use by specific guest OS instances.
2. Network Access Protection enforces health and remote access policies to keep non-compliant and unauthorised clients off the internal and external network.
3. Vastly improved Terminal Services with HTTPS tunnelling and RemoteApp double-click launch of server-hosted Windows applications.
4. HTTPS tunneling and TCP socket sharing allow publishing of services through a small number of restricted, heavily monitored sockets.
5. A new "majority quorum" model for fail-over clustering lets you assign a vote to each cluster node and also to a shared storage device, assuring that if any one fails there is still a majority to constitute a quorum. The quorum disk in a two-node cluster is no longer a single point of failure.
6. Server Core provides a stripped-down server install with only essential services, giving virtual guests a smaller footprint and streamlining the operation and security of main Windows roles such as DNS, DHCP, and file serving.
7. Next Generation TCP/IP dramatically improves network performance through regular, automatic adjustments of the receive window size per connection, though support is currently limited to Windows Server 2008 and Vista. Also big in Next Generation TCP is the offload of TCP processing to supporting NICs, so the server's CPU can concentrate on server processes instead of communication processes.
8. Transactional NTFS lets you define transactions for server-level operations (e.g., to copy files to a directory, create a registry entry, and register a DLL) to ensure that they all complete or that the entire operation rolls back.
9. Restartable Active Directory Domain Services allows you to stop and start directory services without rebooting the domain controller or interrupting other Active Directory services. That means Windows Server 2008 can still handle DNS, DHCP, WINS, and all other requests during directory maintenance.
10. Object-oriented PowerShell replaces the DOS box for command-line administration and scripting.
Honorable mentions: Also noteworthy are Windows Server 2008's extremely strong, flexible, and standards-compliant encryption and security, .NET 3.0 implemented in ASP.NET and throughout the user level, self-healing NTFS, read-only domain controllers, multi-path I/O, and WS08's full support for enhanced features in Windows Vista.