I read somewhere that millions of consumers who buy HD televisions actually do not even receive HD signals in their homes. I also noticed through my own eye-witness experience that even some of those who do have HD unfortunately have not properly installed their TVs to the correct settings to maximize the high-definition potential of their screens. It annoys me. Sometimes, even in electronic stores like Best Buy the walls of TV monitors on display are not showing true HD quality pictures. Once you see the real thing, you notice the difference.
It's like going to a movie theater and seeing the difference between good digital projection and bad, old-school celluloid projection, not the kind in a good theater where the projectionist actually cares enough to properly maintain the equipment and handle the film strips with care. Now that I'm used to pristine digital film, I become easily distracted by out-of-focus, poorly synced celluloid, especially if the reel has scratchmarks, dust, hair, or other impurities after a few showings.
It should also be noted that not all HDs are created equal. There is obvious difference in quality after you've seen the best, just compare (depending on your screen-size and what type of programming you're watching) 120Hz vs. 60Hz, and LEDs vs. LCDs and plasmas, 1080p vs. 1080i, etc.
I hope I don't come across as a technology snob, but once you see it, the differences are stunning, and since the current television sets are made for high resolution, if you don't have HD, you might almost be better off sticking with an old dinosaur TV, you'll probably get a better reception!