Windows Server 2008 incorporates the completely rewritten TCP/IP stack we shipped in Windows Vista and since this is the first server release with the stack we'd like to ask our readers to load up Windows Server RC1 and see if your applications are ready.
The new features (compared to Windows Server 2003) that may pose potential compatibility issues include: removed filter hooks, receive window auto tuning, dual IP layer architecture for IPv6, compound TCP, ECN support, default strong host model, easier kernel mode programming, extensive protocol offload.
Windows Server 2008 is also the very first Microsoft server product to have the firewall ON by default. Third party applications that are not aware of this behavior may break.
-- Ari and Katarzyna