..i.e. lack of documentation.
Today I installed Microsoft Hyper-V on our new test server, a HP DL380 with 32GB memory and plenty of disk (800GB in RAID 5 and a hot spare). The install was quick and easy, but getting it on net was not. With a new router/firewall, configuration new to us, we thought it was causing all the "pain" and struggled for hours until we decided enough is enough, let's continue tomorrow.
Luckily, surfing the net, I stumbled upon the solution to our grieve. Microsoft Hyper-V has its firewall enabled per default (yes, I know, I should have known it was). To managed the hidden features of Microsoft Hyper-V, you have to use netsh from the command prompt, and to disable the firewall (remember I said, we're using an external firewall, and hence we don't need the on-board firewall) all you have to do is:
netsh firewall set opmode disable
Hopefully this information will help you when you install your Hyper-V, and hopefully Microsoft will add this to their FAQ.