I am in Seattle at the moment at an internal Microsoft conference. Yesterday I attended a presentation by one of the product managers for Hyper-V where he really stressed a point that people ask me all the time, he also explained to everyone the reasoning behind the decisions made. The 'issue' people have is regarding typical laptop functionality that is lost when you enable Hyper-V on a laptop with Windows Server 2008 installed. Basically, when you enable Hyper-V on a laptop you lose the Sleep and Hibernate power features that you had before enabling Hyper-V. These features completely disappear when you install the role and you cannot re-enable them, nor hack the registry in order to get them back; Windows Server 2008 is pretty much just told to forget all about these power-saving features. So, when you close the lid of your laptop nothing happens. When you battery runs out of power the machine will shutdown or turn off, there is no gracefully power down into hibernation mode saving your desktop session and everything else that you were doing at the time.
As you can see in the screenshot below on the left, the only options available are "Do nothing" or "Shut down" if I close the lid on my laptop whereas before enabling Hyper-V I had the option "Sleep" in the list as well. In the second screenshot on the right you can see that for "Critical battery action" the only option available is "Shut down" which before would have been "Hibernate".
A lot of people tell me that they do not like this because the Sleep mode of laptops is super useful, especially when moving around the office when you need to carry the laptop with you. The reasoning behind taking the decision to disable this functionality is simple; Windows Server 2008 is a server operating system and was not designed to accommodate all the functionality that laptop users want or need. If you have a server running 20 virtual servers, you are extremely unlikely to ever need to hibernate it at all, or even want to. And even if you could it would probably cause some bizarre things to happen inside the VMs as the hardware devices powered off.
So, if you really need to be able to put your laptop to sleep, then you are better off running Windows Vista, or dual-booting your machine with Windows Server 2008 and Windows Vista. Me? I am happy to accept the loss of this functionality as I find Hyper-V so useful. I have set the laptop to "Do nothing" if I close the lid and it shuts itself down when the battery goes flat.