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.
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.
"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."
I'd say the last one indicates GPT is required for an UEFI system. "using all UEFI capabilities" is somewhat vague though, are there specific capabilities you would like to see used?
Also, note that Windows 7 x64 will do GPT on UEFI systems too. And also Vista SP1, and Server 2008.