I wrote recently about IPv6 and new features in "Vista" and "Longhorn" both of which share a brand new IP stack (IPv4 and IPv6 combined), there is another TechNet article with additional details here.  One of the most often talked about features is Receive-Side Scaling (RSS), which will alleviate issues with a single CPU handling all the load, as the traffic can be spread across CPUs in an SMP system allowing more incoming traffic without adding servers - you will need RSS aware network cards.

Also I like the fact that IPsec and the Windows Firewall are configurable from a single tool - which should make adoption and deployment easier.  Also IPsec and clustering has been improved greatly reducing the timeout period that we have today.