Next Generation TCP/IP
Longhorn has a completely revamped TCP/IP stack called Next Generation TCP/IP (NGTCP). NGTCP's biggest improvement is the auto-adjusting of the receive window. The Receive Window Auto-Tuning capability determines the optimal receive window size per connection by measuring the bandwidth-delay product (the bandwidth multiplied by the latency of the connection) and the application retrieval rate. It then automatically adjusts the maximum receive window size on a regular basis. This can actually increase performance by orders of magnitude under the right conditions.
Of course, there is a catch: To take advantage of this feature, both servers involved in the communication have to use NGTCP. Today, that is limited to Vista and Longhorn.
Another seriously cool feature in NGTCP is the transport offload engine (TOE). The TOE lets the server offload TCP processing to the NIC, so the server's CPU can concentrate on server processes instead of communication processes. Offloaded processes include checksum calculations for both TCP and UDP, IP security authentication and encryption, segmentation of large TCP packets, and the TCP stack itself. But again there's a catch: You need specialized NICs.
What's still missing
Two features didn't make it into Longhorn: WinFS and SysInternals.
WinFS was going to provide Windows with a SQL Server back end to allow free indexing and instant searching of documents and other files on the file system. This addition was going to change the face of the operating system completely. Currently, the registry and other underpinnings are all part of the OS and work together as a single contiguous unit. However, with WinFS, it would make the OS a SQL client just like any other. Conceptually, Windows wouldn't be any different than Internet Information Server or any other compiled application you write against the database.
This alteration not only would have made DBAs the new gods of the OS, it would have separated the functional and data layers of Windows. And eventually, that could have led to a completely different paradigm for backing up, hot-fixing, service packing, disaster recovery, you name it. Admins would still benefit from WinFS, and so many are still waiting for it.
Likewise, many admins were looking forward to seeing the SysInternals tools roll into Windows when Microsoft acquired the company in 2006 -- especially native versions of the troubleshooting capabilities of ERD Commander. (For the record, Microsoft never said it would integrate SysInternals' tools into Longhorn.) Unfortunately, that didn't happen. And even though Longhorn has a Repair Your Computer option in the setup program (similar to Vista), it still doesn't give you the capabilities that ERD Commander does.
Despite what got cut, Microsoft has delivered improved uptime, stability, and speed in Longhorn. Not all features work out of the box, and not everyone will take advantage of all the changes. But with the Longhorn release, Windows Server is no longer the bottleneck to scaling your applications.