The first thing I noticed is that you actually have to forestprep and domainprep your domain in preparation for a 2008 R2 Domain Controller.
Windows Server 2008 R2 intriduces PowerShell 2.0, which significantly enhances the earlier version with the inclusion of more than 240 new pre-built cmdlets as well as a new graphical user interface (GUI) that adds professional-level development features for creating new cmdlets. The new GUI includes colored syntaxing, new production script debugging capabilities, and new testing tools.
PowerShell 2.0 also has a deeper reach than its predecessor, with enhanced support available on both Windows 7 and the Server Core role (which previously could not run PowerShell).
The current Active Directory command line functions have been replaced with 85 Active Directory related cmdlets. It is also now installed out of the box, and no more installing it from the features portion of Server Manager.
2008 R2 Functional Level
Active Directory Domain Services will be sporting a new forest functional level. This will no doubt bring a lot of new features with it, and I’m not sure if any or all of these features listed will require that functional level.
Active Directory Recycle Bin
Deleted Active Directory items can be found in a recycle bin and can be restored as such. I believe this will require the R2 functional level.
Offline Domain Joining
Windows 7 machines can have domain joining automated offline with XML files. Domain Join (djoin.exe) is the tool to do so.
Authentication Assurance provides an authentication mechanism that allows administrators to map specific certificates to security groups using certificate policies. This is described in more detail here. http://blogs.technet.com/activedirectoryua/archive/2008/11/21/authentication-mechanism-assurance-in-windows-server-2008-r2.aspx
Active Directory Administrative Center
The Administrative Center replaces the current Active Directory Users and Computers (ADUC) MMC Snap-in (dsa.msc).
The new Management Console is a graphical shell for Powershell. The cool thing is that after clicking together your commands, the administrative center shows the corresponding Powershell command on the screen and then executes it. Similar to Exchange 2007 Management Tools and Virtual Machine Manager 2008.
Best Practices Analyzer
Accompanying the Active Directory Administrative Center is the Active Directory Best Practices Analyzer (ADBPA), which will help Active Directory administrators to correct Active Directory problems proactively and compare Active Directory performance with previously made baselines. Similar to Exchange’s BPA.