Windows Server 2008 R2 brings a lot of improvements in the virtualization area. Some of the top improvements include:

1. Hot-Add VM Storage

Windows Server 2008 R2 Hyper-V supports hot plug-in and hot removal of disk storage in the format of Virtual Hard Drive (VHD) files and/or pass-through disks. This can be done while a VM is running, and allows administrators to meet changing workload requirements.

The rest is on Petri's blog :