I have noticed the same thing. In my opinion, what we are seeing and hearing is the fact that the HD signal uses little or no video and audio compresson. Thus, the video and audio dynamic range are much, much greater than a standard NTSC broadcast. I simply turn up the brightness about 5 notches and increase the volume when watching these channels. It is a bit of a pain when switching back to a standard baseband channel. I think we will be in this transition period for several more years. For now, HD is worth the extra trouble; I remember having only 4 black & white grainy channels and mono audio through a cheap speaker.
