Stewart, there are many reasons why some stations look better than others. Stations that broadcast in "True" HD, both 720p and 1080i, have excellent picture. The key word here is "True." Many stations that have the "HD" badge take a lot of existing "crap" and upres it to HD. The old saying is very true here: "Crap in, crap out." TNT stuff looks horrible (to me) because they take standard-def material, upres it, and they compress the hell out of it to make an additional 5+ minutes of commercial time every hour. All commercial channels do time-compression on most of their programming. Basically, they take a 60 minute program and they speed it up by 10 to 15%, then they process the audio so the tone and pitch is corrected back to "normal". But the "speed-up" process technically drops frames every so often instead of making everything run in fast-motion. That's how they get an extra 5+ minutes of commercial time for a 60 minute show. Watch horizontal motion on most cable programs or movies. You can see skipping or jumping of the action. This is caused by the dropped video frames.
Then there is the compression that our satellite or cable companies do to squeeze the bandwidth so it can be easily broadcast. There can be separate compression for different channels, which is why some stations look worse than others. Keep in mind that the more stations that need airspace, the more compression we'll need to fit them into the limited bandwidth. This will be a real issue as more HD channels hit the airwaves.
So you see, there are many factors involved in whether we see a good or bad picture on our TV. If you watch the David Letterman or The Tonight Show, both look excellent. DiscoveryHD looks great as does most PBS HD programs. All of these shows originate in "True" HD. Some are 720 and some are 1080, but all look great.