We like to keep systems upgraded here and have an all 2008 Server Standard (with the exception of a few systems in which the vendor only supports 2003R2) fleet.
For the most part, none of the systems are particularly complex, nor do we have an overwhelming number of systems. A bunch of AD/DNS/etc. systems, some SQL Server boxes, various non-mission-critical servers, and an Exchange server.
Virtualization & HA is all done within VMWare ESX Server so we don't really find the Hyper-V stuff or OS-based clustering all that interesting.
Given that we're all comfy and cozy on 2008 and aren't really taxing the systems we have is there a real need to spend the person-power to upgrade to 2008 R2? Should we just wait the n-number of years until the next major release of Windows Server? Assume all of our hardware is relatively new and is expected to be in-service for the useful life of the hardware.
EDIT: We're running XP Pro on 100% of the desktops but as they age out will be replacing them with Win7. Our group/domain policy is very simple and (in our testing) wouldn't require any changes for Win 7. No foreseeable plans to massively change our office/workstation/etc. topology.