The most interesting new feature to me is built-in seamless windows on Microsoft Windows guest OS, after booting your VM just type CTRL-L, the Windows desktop will disappear and all windows will be popped out in your host OS. It would be wonderful to be able to switch between host and guest applications with the ALT-Tab but when you try to do that you’ll see that all guest applications are seen as a single VirtualBox process, quite obvious. If you want to see guest applications as different host processes you’ll have to connect to the guest OS with a seamless RDP client.
Also the shared clipboard works much better but the changelog is really long thus check it out.
This is really a great piece of software!