Just a quick heads up; as I was setting up a 2012 lab today, on Virtualbox, I just went on auto-pilot and installed the Guest Additions. They install just fine, but after a reboot, the server was useless. Started hanging pretty much regardless of what I tried to do.
Not sure what the problem is; but I did see that the Desktop Window service was taking quite a bit of CPU, so it might be a graphics problem.
Anyway. Uninstalled, and all is well. Missing a few functions, but it works.