Microsoft will require computers with the "Designed for Windows 8" logo to use UEFI with secure boot (which will only allow signed software to run on the device) enabled by default.
Fonte: link
Windows 8 provides additional support for security features to help to prevent unauthorized firmware, operating systems, or UEFI drivers from running at boot time. These features include Secure Boot and support for factory-encrypted drives. UEFI firmware is required to support large disks (over 2 TB). This firmware can also provide faster boot and resume speeds, by reading and using data more efficiently.
Fonte: link
"Windows cannot be installed to this disk. The selected disk has an MBR partition table. On EFI systems, windows can only be installed to GPT disks."
Fonte: link
Eu diria que o último indica que o GPT é exigido para um sistema UEFI. "usando todos os recursos da UEFI" é um pouco vago, existem recursos específicos que você gostaria de ver usados?
Além disso, observe que o Windows 7 x64 também executará a GPT em sistemas UEFI. E também o Vista SP1 e o Server 2008.