HD is still in its infancy.However,apart from the sports that you now can get in HD (MLB,NFL,NBA) you can also get some great stuff on Discovery HD,National Geographic HD,PBS HD and other channels.I,personally,love nature documentaries set in Africa.The sight of an elephant,a rhino (no,not a RINO) or a giraffe in HD is something to behold,IMO.
As time goes on there'll be more and more worthwhile stuff being shown in HD.