"fortunately including TNT, as my wife likes Law and Order"
Are Law and Order reruns in HD or are you watching 4:3 TV in HD?
It's full widescreen high-rez HDTV; switching between this and the SD TNT channel (which is also running on Comcast) shows the difference instantly.
Most of the alphabet networks (CBS, NBC, ABC) run their ordinary shows in SD which even on their "HD" feeds is as you describe, just a 4:3 SD picture displayed in 720p rez.