Top 3 things to try in VirtualBox 3.1
Here are the top 3 things I personally think VirtualBox 3.1 will be popular for:
Teleportation – a new term coined by VirtualBox team for live migration. This allows you to migrate a live VM environment from one host to another. Traditionally being a feature of only the most advanced server-grade virtualization solutions, live migration is surely a welcome feature in VirtualBox. For those who haven't tried it yet, live VM means it's up and running and live migration implies your VM keeps functioning and is fully accessible throughout the migration – an obvious improvement from previous "shut down VM on host1, start it up on host2".
Improved management of VM states on the snapshot level – apparently, you can now restore your VM state from any snapshot and not just the latest one. Starting with VirtualBox 3.1, it is also possible to branch snapshots out – take snapshots of a any given snapshots.
Paravirtualized network adapters (virtio) support – this is a great feature which allows you to use the well-known virtio network adapter by KVM instead of a standard driver software emulating the hardware of a network card. In short, this allows your VMs to use a highly optimized software solution for virtual network cards.