The TiVoHD and all Series 3 TiVos, cannot display MPEG4/H.264 channels. The TiVoHD most likely has the hardware capabilities, but TiVo chose not to update the SW. Even the Series 4 Premieres needed a SW update after Cox launched the MPEG4/H.264 channels, they also were not capable of displaying Copy Protected H.264 channels until May 2012.
Premieres and Roamios work fine now with the Contour (formerly Plus Pak) MPEG4/H.264 channels.
Cox CableCARD and Retail Devices - The link is a little outdated but lists which devices cannot access the Plus Pak
Also, the Intel Core i3/5/7 and their integrated Intel GPU HD Graphics cannot display Copy Protected H.264 channels in WMC 7.
(I'm not sure about 3rd Gen Intel Core i CPUs though, 1st and 2nd Gen will not work without a dedicated graphics card in WMC 7.)