Branch Office Support - increasingly, enterprises want to deploy systems to branch offices, for a variety of reasons including local productivity and resilience. R2's branch office features will allow you to consider a branch office nothing more than a lazy cache of something held (and managed, and backed up and analysed!) at a central hub site. To achieve this, R2 has major updates to FRS (including a differencing file transfer protocol) and DFS.
Active Directory Federation - this extends AD to support web single sign-on as well as B2B/B2C systems. For more info on ADFS see the .NET Show epidode with David McPherson and Don Schmidt.
Storage - R2 has a number of features in the storage area, including real filestore quotas, simple SAN support, the abilty to deny storing inappropriate content (where the admin determines what is appropriate) etc.
With Windows 2003 SP1 nearing completion, 2005 looks like seeing quite a lot of new technology from Microsoft, including the first serious beta of Longhorn, Windows 2003 SP1 RTM and R2 RTM, as well as other smaller programs (e.g. SUS, MACS, etc).