I have an HDTV and what you say is true but a few SD channels look almost as good as HD. White Springs TV looks crappy while PBS SD looks very close to HD.
The PBS stuff has improved a lot in the last few months, Best part of a year ago, some of it was absolutely TERRIBLE, when viewed on an HDTV. Particularly some of those Masterpiece shows that originate in England. The older ones in SD looked OK on my old analog TV, but looked absolutely terrible on my HDTV. My guess is that the original must have been converted from PAL, and lost some definition there, then lost more when scaled up to the 1080 HDTV. The dimensions just didn't match up to the dimensions of the TV. But recent shows that must come from HD originals look great, whether viewed on the SD or HD channel.
I think it's largely a question of whether the dimensions of the screen are close to an even multiple of the video. I've actually found that some SD looks better when put into a side by side PIP than it does if you run it full screen. Not because you're exxagerating the imperfections, but because the PIP just happens to often have closer to the proper dimensions.
The problem of losing definition upon scaling up SD video to HDTV screen is also particularly bad at times with my OTA channel. The local channel hasn't figured out yet how to superimpose their SD text crawlers onto a HD broadcast, and when we have storm information (closings and such) displayed at the bottom of the screen, it used to be that you couldn't read it. Apparently someone complained, so when we have storms, they now switch the HD broadcast to SD, so that the text matches up better.