Few things I've seen come up here recently with R2, so heads up:
  1. Forest and Domain functional level - must be at least Windows Server 2003.  This differs from OCS which required Windows Server 2000.
  2. 32bit DCs - R2 binaries are x64 only so consider this as you run AD prep steps.  While you don’t need to run directly on a DC, ideally you have an x64 member server in the Forest root domain to run Schema and Forestprep.  This can be a box you build out for your first R2 server.
  3. Global Settings default in Config Container - R2 wants to store its global settings in AD's config naming context rather than the domain naming context (which LCS did and OCS recommended).  For Greenfield this isn't an issue but if you're moving from OCS or LCS you may want to review a script we've released to move global settings.  Here's the script and doc on this - note this isn't required but if you want to do it has to be done *before* running R2 schema prep.  I wrote a post on this awhile back for OCS; we're working on updating for R2.